[3.12] Fix grammar typo in Doc/c-api/arg.rst (GH-130741) (#130758)

Fix grammar typo in `Doc/c-api/arg.rst` (GH-130741)
(cherry picked from commit 37145cb)

Co-authored-by: Arijit Kumar Das <[email protected]>

Author: miss-islington

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`.