Docs: Fix documentation for "PyTypeObject Definition" (GH-130936)

The structure definition for `PyTypeObject` is in `Include/cpython/object.h`,
not in `Include/object.h`.

Author: Yuki Kobayashi

Doc/c-api/typeobj.rst

@@ -473,7 +473,7 @@ PyTypeObject Definition
 -----------------------
 
 The structure definition for :c:type:`PyTypeObject` can be found in
-:file:`Include/object.h`.  For convenience of reference, this repeats the
+:file:`Include/cpython/object.h`.  For convenience of reference, this repeats the
 definition found there:
 
 .. XXX Drop this?