gh-157242: Fix typo in _PyBytes_Resize() (#157619)

PyBytes_AsString() should be used to modify the object, not
PyBytesWriter_GetData().

Author: vstinner

Doc/c-api/bytes.rst

@@ -249,7 +249,7 @@ called with a non-bytes parameter.
 
    While bytes objects are usually immutable in Python, this special C API
    allows mutating a bytes object in-place. The returned bytes object can still
-   be mutated using :c:func:`PyBytesWriter_GetData`; except if *newsize* is
+   be mutated using :c:func:`PyBytes_AsString`; except if *newsize* is
    zero in which case it returns the immutable empty bytes string.
 
    .. soft-deprecated:: 3.15