Docs: Fix incomplete sentence in tp_itemsize documentation (GH-152381)

(cherry picked from commit 0a21a24)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>

Author: da-woods

Doc/c-api/type.rst

@@ -639,7 +639,7 @@ but need extra remarks for use as slots:
    in the following situations:
 
    - The base is not variable-sized (its
-     :c:member:`~PyTypeObject.tp_itemsize`).
+     :c:member:`~PyTypeObject.tp_itemsize` is zero).
    - The requested :c:member:`PyType_Spec.basicsize` is positive,
      suggesting that the memory layout of the base class is known.
    - The requested :c:member:`PyType_Spec.basicsize` is zero,