Fixed a typo in arg.rst

Fix typo: 'extensions functions'  -> 'extension functions' in documentation.

Author: ArijitKD

Doc/c-api/arg.rst

@@ -5,7 +5,7 @@
 Parsing arguments and building values
 =====================================
 
-These functions are useful when creating your own extensions functions and
+These functions are useful when creating your own extension functions and
 methods.  Additional information and examples are available in
 :ref:`extending-index`.