Update comment at top of index_create

I neglected to update it in commit f4c4335a.

Michael Paquier

src/backend/catalog/index.c

@@ -670,6 +670,7 @@ UpdateIndexRelation(Oid indexoid,
  * concurrent: if true, do not lock the table against writers.	The index
  *		will be marked "invalid" and the caller must take additional steps
  *		to fix it up.
+ * is_internal: if true, post creation hook for new index
  *
  * Returns the OID of the created index.
  */