Update README.rst

Fix link to Descripyion RDoc

Author: iamsimakov

README.rst

@@ -999,7 +999,7 @@ the default version is used instead.
 ========
 
 The default markup language is `RDoc
-<http://rdoc.rubyforge.org/RDoc/Markup.html>`_. It can be changed in
+<https://rdoc.github.io/rdoc/RDoc/Markup.html>`_. It can be changed in
 the config file (``config.markup=``) to one of these:
 
 Markdown