Reference --shortcut-mod from shortcuts list

Fixes #1681 <https://github.com/Genymobile/scrcpy/issues/1681>

Suggested-by: Moritz Schulz <moritzleni@gmail.com>

app/scrcpy.1

@@ -222,7 +222,7 @@ Default is 0 (automatic).\n
 .SH SHORTCUTS
 
 In the following list, MOD is the shortcut modifier. By default, it's (left)
-Alt or (left) Super, but it can be configured by \-\-shortcut-mod.
+Alt or (left) Super, but it can be configured by \-\-shortcut-mod (see above).
 
 .TP
 .B MOD+f

app/src/cli.c

@@ -203,7 +203,7 @@ scrcpy_print_usage(const char *arg0) {
         "\n"
         "    In the following list, MOD is the shortcut modifier. By default,\n"
         "    it's (left) Alt or (left) Super, but it can be configured by\n"
-        "    --shortcut-mod.\n"
+        "    --shortcut-mod (see above).\n"
         "\n"
         "    MOD+f\n"
         "        Switch fullscreen mode\n"