Fix wrong file path in header comment

Header comment of shm_mq.c was mistakenly specifying path to shm_mq.h.
It was introduced in ec9037df.  So, theoretically it could be
backpatched to 9.4, but it doesn't seem to worth it.

src/backend/storage/ipc/shm_mq.c

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * src/include/storage/shm_mq.h
+ * src/backend/storage/ipc/shm_mq.c
  *
  *-------------------------------------------------------------------------
  */