Standardise on "reStructuredText" over "reST" in documentation

doc/development/tutorials/extending_syntax.rst

@@ -138,7 +138,7 @@ The simplest thing you can do is to call the
 :meth:`.Sphinx.add_role` and :meth:`.Sphinx.add_directive` methods,
 which is what we've done here.
 For this particular call, the first argument is the name of the role/directive itself
-as used in a reST file.
+as used in a reStructuredText file.
 In this case, we would use ``hello``. For example:
 
 .. code-block:: rst