Fix a/an grammar uses (#907)

"A user" respects the same rules as "a union" in these examples:
https://www.wikihow.com/Sample/An-A-Usage

integrations/pkg_config_pc_files.rst

@@ -6,7 +6,7 @@
 If you are creating a Conan package for a library (A) and the build system uses *.pc* files to locate
 its dependencies (B and C) that are Conan packages too, you can follow different approaches.
 
-The main issue to solve is the absolute paths. When an user installs a package in the local cache,
+The main issue to solve is the absolute paths. When a user installs a package in the local cache,
 the directory will probably be different from the directory where the package was created. This could be
 because of the different computer, the change in Conan home directory or even a different user or channel:
 

reference/conanfile/attributes.rst

@@ -145,7 +145,7 @@ channel than the current package, which could be achieved with something like:
         def requirements(self):
             self.requires("Say/0.1@%s/%s" % (self.user, self.channel))
 
-Only package recipes that are in the conan local cache (i.e. "exported") have an user/channel assigned.
+Only package recipes that are in the conan local cache (i.e. "exported") have a user/channel assigned.
 For package recipes working in user space, there is no current user/channel. The properties ``self.user``
 and ``self.channel`` will then look for environment variables ``CONAN_USERNAME`` and ``CONAN_CHANNEL``
 respectively. If they are not defined, an error will be raised unless ``default_user`` and ``default_channel``
@@ -275,7 +275,7 @@ values:
         options = {"shared": [True, False],
                    "option1": ["value1", "value2"],}
 
-Values for each option can be typed or plain strings, and there is an special value, ``ANY``, for
+Values for each option can be typed or plain strings, and there is a special value, ``ANY``, for
 options that can take any value.
 
 The attribute ``default_options`` has the purpose of defining the default values for the options

reference/env_vars.rst

@@ -332,7 +332,7 @@ or declared in command line when invoking :command:`conan install` to reduce the
 
     $ conan install . -e CONAN_RUN_TEST=0
 
-See how to retrieve the value with :ref:`tools.get_env() <tools_get_env>` and check an use case
+See how to retrieve the value with :ref:`tools.get_env() <tools_get_env>` and check a use case
 with :ref:`a header only with unit tests recipe <header_only_unit_tests_tip>` while cross building.
 
 See example of build method in ``conanfile.py`` to enable/disable running tests with CMake:

reference/tools.rst

@@ -778,7 +778,7 @@ tools.run_in_windows_bash()
 
     def run_in_windows_bash(conanfile, bashcmd, cwd=None, subsystem=None, msys_mingw=True, env=None)
 
-Runs an unix command inside a bash shell. It requires to have "bash" in the path.
+Runs a UNIX command inside a bash shell. It requires to have "bash" in the path.
 Useful to build libraries using ``configure`` and ``make`` in Windows. Check :ref:`Windows subsytems <windows_subsystems>` section.
 
 You can customize the path of the bash executable using the environment variable ``CONAN_BASH_PATH`` or the :ref:`conan.conf<conan_conf>` ``bash_path``

uploading_packages/bintray/conan_center_guide.rst

@@ -108,10 +108,10 @@ Recipe Quality
         # Package it
         self.copy("license*", dst="licenses",  ignore_case=True, keep_path=False)
 
-- **Invalid configurations:** There is an special exception, ``conans.errors.ConanInvalidConfiguration`` to be launched
+- **Invalid configurations:** There is a special exception, ``conans.errors.ConanInvalidConfiguration`` to be launched
   from ``configure()`` function in a recipe if the given configuration/options is known not to work. This way the recipe
   owner can declare an invalid configuration and consumers (e.g. CI tools like ``conan-package-tools``) will be able to
-  handle it.  
+  handle it.
 
 
 CI Integration