pg_resetxlog: Make --help consistent with man page

Use "MXID" as placeholder for -m option, instead of just "XID".

src/bin/pg_resetxlog/pg_resetxlog.c

@@ -1038,7 +1038,7 @@ usage(void)
 	printf(_("  -e XIDEPOCH      set next transaction ID epoch\n"));
 	printf(_("  -f               force update to be done\n"));
 	printf(_("  -l XLOGFILE      force minimum WAL starting location for new transaction log\n"));
-	printf(_("  -m XID,XID       set next and oldest multitransaction ID\n"));
+	printf(_("  -m MXID,MXID     set next and oldest multitransaction ID\n"));
 	printf(_("  -n               no update, just show extracted control values (for testing)\n"));
 	printf(_("  -o OID           set next OID\n"));
 	printf(_("  -O OFFSET        set next multitransaction offset\n"));