Add -G to Available Argument List

-G is the way I primarily boot my ships and yet that flag has not been in the usage notes for some time (ever?). Adding it should assist new pilots in getting running and avoid questions of "I booted my ship but it took an hour and now I'm a comet" which sometimes (used to, at least) come w/ new pilots attempting to `-K kEy-S741nG`.

pkg/urbit/daemon/main.c

@@ -446,6 +446,7 @@ u3_ve_usage(c3_i argc, c3_c** argv)
     "-d            Daemon mode; implies -t\n",
     "-e url        Ethereum gateway\n",
     "-F ship       Fake keys; also disables networking\n",
+    "-G            Boot using key-string from key file\n",
     "-g            Set GC flag\n",
     "-i jam_file   import pier state\n",
     "-j            Create json trace file in .urb/put/trace\n",