Remove references to 88colour support

Tmux doesn't directly support terminals with 88 colours directly anymore.

tmux.c

@@ -60,7 +60,7 @@ __dead void
 usage(void)
 {
 	fprintf(stderr,
-	    "usage: %s [-28lquvV] [-c shell-command] [-f file] [-L socket-name]\n"
+	    "usage: %s [-2lquvV] [-c shell-command] [-f file] [-L socket-name]\n"
 	    "            [-S socket-path] [command [flags]]\n",
 	    __progname);
 	exit(1);