[3.13] Docs: Replace "encoder" with "decoder" in Doc/c-api/codec.rst (pythonGH-140613) (python#140662)

Docs: Replace "encoder" with "decoder" in `Doc/c-api/codec.rst` (pythonGH-140613)
(cherry picked from commit d12086e)

Co-authored-by: RayXu <140802139+F18-Maverick@users.noreply.github.com>

Author: miss-islington

Doc/c-api/codec.rst

@@ -39,7 +39,7 @@ Codec registry and support functions
    *object* is passed through the decoder function found for the given
    *encoding* using the error handling method defined by *errors*.  *errors* may
    be ``NULL`` to use the default method defined for the codec.  Raises a
-   :exc:`LookupError` if no encoder can be found.
+   :exc:`LookupError` if no decoder can be found.
 
 
 Codec lookup API