Fix typos

libcxx/docs/ReleaseNotes/18.rst

@@ -126,7 +126,7 @@ Improvements and New Features
 
 - AddressSanitizer annotations have been added to ``std::basic_string``.
   These annotations are enabled for all allocators by default.
-  It's only enable for long string, string using the small buffer optimization are not annotated.
+  It's only enabled for long strings, strings using the small buffer optimization are not annotated.
 
 - The libc++ source code has been formatted with ``clang-format``. This
   `discourse thread <https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all>`_