Fix documentation for gcvisitobjects_t

Documented behaviour is the exact opposite of the real behaviour.

See comment in the code

https://github.com/python/cpython/blob/ad3bbe8fbcc9045c436b0c5e49b1aec018c59c2d/Include/cpython/objimpl.h#L92-L93

and code

https://github.com/python/cpython/blob/ad3bbe8fbcc9045c436b0c5e49b1aec018c59c2d/Python/gc.c#L2405

Author: da-woods

Doc/c-api/gcsupport.rst

@@ -277,7 +277,7 @@ the garbage collector.
 
    Type of the visitor function to be passed to :c:func:`PyUnstable_GC_VisitObjects`.
    *arg* is the same as the *arg* passed to ``PyUnstable_GC_VisitObjects``.
-   Return ``0`` to continue iteration, return ``1`` to stop iteration. Other return
+   Return ``1`` to continue iteration, return ``0`` to stop iteration. Other return
    values are reserved for now so behavior on returning anything else is undefined.
 
    .. versionadded:: 3.12