Update code comment re: location of struct _is. (GH-20067)

diff --git a/Include/pystate.h b/Include/pystate.h
index 34cad02..bae4407 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -18,7 +18,7 @@
 
 /* struct _ts is defined in cpython/pystate.h */
 typedef struct _ts PyThreadState;
-/* struct _is is defined in internal/pycore_pystate.h */
+/* struct _is is defined in internal/pycore_interp.h */
 typedef struct _is PyInterpreterState;
 
 PyAPI_FUNC(PyInterpreterState *) PyInterpreterState_New(void);