Closes #17725: small grammar fix.

Doc/extending/index.rst

@@ -5,12 +5,12 @@
 ##################################################
 
 This document describes how to write modules in C or C++ to extend the Python
-interpreter with new modules.  Those modules can define new functions but also
-new object types and their methods.  The document also describes how to embed
-the Python interpreter in another application, for use as an extension language.
-Finally, it shows how to compile and link extension modules so that they can be
-loaded dynamically (at run time) into the interpreter, if the underlying
-operating system supports this feature.
+interpreter with new modules.  Those modules can not only define new functions
+but also new object types and their methods.  The document also describes how
+to embed the Python interpreter in another application, for use as an extension
+language.  Finally, it shows how to compile and link extension modules so that
+they can be loaded dynamically (at run time) into the interpreter, if the
+underlying operating system supports this feature.
 
 This document assumes basic knowledge about Python.  For an informal
 introduction to the language, see :ref:`tutorial-index`.  :ref:`reference-index`