Fix comment in walsender_private.h

All the members of the stucture are protected by the spinlock WalSnd,
but a comment referred to "replyTime" and "latch" as not being in the
set of what gets protected, contrary to what walsender.c does.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACWE_7srye4_GZ=N=-rD=qr2WHL9GZrMnhWJOJ5RdnNS2A@mail.gmail.com

src/include/replication/walsender_private.h

@@ -65,7 +65,7 @@ typedef struct WalSnd
 	 */
 	int			sync_standby_priority;
 
-	/* Protects shared variables shown above. */
+	/* Protects shared variables in this structure. */
 	slock_t		mutex;
 
 	/*