Fix erroneous cross-reference in comment.

Seems to have been introduced in commit c219d9b0a.  I think there indeed
was a "tupbasics.h" in some early drafts of that refactoring, but it
didn't survive into the committed version.

Amit Kapila

src/include/access/htup_details.h

@@ -165,7 +165,7 @@ struct HeapTupleHeaderData
 	/* MORE DATA FOLLOWS AT END OF STRUCT */
 };
 
-/* typedef appears in tupbasics.h */
+/* typedef appears in htup.h */
 
 #define SizeofHeapTupleHeader offsetof(HeapTupleHeaderData, t_bits)