[3.14] Docs: Fix a grammatical error in Doc/c-api/codec.rst (GH-140612) (#140616)

Docs: Fix a grammatical error in `Doc/c-api/codec.rst` (GH-140612)
(cherry picked from commit 33b2ca8)

Co-authored-by: RayXu <[email protected]>

Author: miss-islington

Doc/c-api/codec.rst

@@ -7,7 +7,7 @@ Codec registry and support functions
 
    Register a new codec search function.
 
-   As side effect, this tries to load the :mod:`!encodings` package, if not yet
+   As a side effect, this tries to load the :mod:`!encodings` package, if not yet
    done, to make sure that it is always first in the list of search functions.
 
 .. c:function:: int PyCodec_Unregister(PyObject *search_function)