Fix typo in comment

Commit 98e675ed7a accidentally mistyped IDENTIFY_SYSTEM as
IDENTIFY_SERVER. Backpatch to all supported branches.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/68138521-5345-8780-4390-1474afdcba1f@gmail.com

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -359,7 +359,7 @@ libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli)
 						pchomp(PQerrorMessage(conn->streamConn)))));
 	}
 	/*
-	 * IDENTIFY_SERVER returns 3 columns in 9.3 and earlier, and 4 columns in
+	 * IDENTIFY_SYSTEM returns 3 columns in 9.3 and earlier, and 4 columns in
 	 * 9.4 and onwards.
 	 */
 	if (PQnfields(res) < 3 || PQntuples(res) != 1)