[3.13] Fix typo in Doc/c-api/code.rst (pythonGH-140586) (python#140592)

Fix typo in `Doc/c-api/code.rst` (pythonGH-140586)
(cherry picked from commit d74a963)

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

Author: miss-islington

Doc/c-api/code.rst

@@ -285,7 +285,7 @@ may change without deprecation warnings.
 
 .. c:function:: Py_ssize_t PyUnstable_Eval_RequestCodeExtraIndex(freefunc free)
 
-   Return a new an opaque index value used to adding data to code objects.
+   Return a new opaque index value used to adding data to code objects.
 
    You generally call this function once (per interpreter) and use the result
    with ``PyCode_GetExtra`` and ``PyCode_SetExtra`` to manipulate