remote-viewer: mention vv-file in help output

The man page already has a description of the vv-file format, but the
--help output didn't mention it how to use it.

References: rhbz#970825

src/remote-viewer-main.c

@@ -104,7 +104,7 @@ main(int argc, char **argv)
           N_("Open connection using Spice controller communication"), NULL },
 #endif
         { G_OPTION_REMAINING, '\0', 0, G_OPTION_ARG_STRING_ARRAY, &args,
-          NULL, "-- URI" },
+          NULL, "URI|VV-FILE" },
         { NULL, 0, 0, G_OPTION_ARG_NONE, NULL, NULL, NULL }
     };