create-usb: Tweak docs for --destination-repo

Make it show up in the help output as --destination-repo=DEST so it's
clear that it takes an argument.

Closes: #1656
Approved by: jlebon

src/ostree/ot-builtin-create-usb.c

@@ -37,7 +37,7 @@ static char *opt_destination_repo = NULL;
 static GOptionEntry options[] =
   {
     { "disable-fsync", 0, 0, G_OPTION_ARG_NONE, &opt_disable_fsync, "Do not invoke fsync()", NULL },
-    { "destination-repo", 0, 0, G_OPTION_ARG_FILENAME, &opt_destination_repo, "Use custom repository directory within the mount", NULL },
+    { "destination-repo", 0, 0, G_OPTION_ARG_FILENAME, &opt_destination_repo, "Use custom repository directory within the mount", "DEST" },
     { NULL }
   };