support spawning dmenu with more than 10 monitors

dwm.c

@@ -1657,7 +1657,7 @@ spawn(const Arg *arg)
 	pid_t pid;
 
 	if (arg->v == dmenucmd)
-		dmenumon[0] = '0' + selmon->num;
+		snprintf(dmenumon, sizeof(dmenumon), "%d", selmon->num);
 
 	if ((pid = fork()) == -1) {
 		fprintf(stderr, "dwm: fork failed: %s\n", strerror(errno));