wam: Use a tuple for the --change-passphrase argument's metavar

Fixes its help entry looking like this:

>  --change-passphrase ADD-ON OLD NEW ADD-ON OLD NEW ADD-ON OLD NEW
>                        Change the passphrase for ADD-ON from OLD to NEW

When it really should look like this:

>  --change-passphrase ADD-ON OLD NEW
>                        Change the passphrase for ADD-ON from OLD to NEW

data/tools/wesnoth_addon_manager

@@ -87,7 +87,7 @@ if __name__ == "__main__":
         help="unpack the file UNPACK as a binary WML packet " +
         "(specify the add-on path with -c)")
     argumentparser.add_argument("--change-passphrase", nargs=3,
-        metavar="ADD-ON OLD NEW",
+        metavar=("ADD-ON","OLD","NEW"),
         help="Change the passphrase for ADD-ON from OLD to NEW")
     args = argumentparser.parse_args()