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

(cherry picked from commit 0a21a24)

Author: da-woods

Doc/c-api/type.rst

@@ -551,7 +551,7 @@ The following functions and structs are used to create
       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,