Add missing message mapping for key command.spy.description in defau…

…lt locale English. (#598)

common/src/main/resources/locale/messages-en_US.properties

@@ -91,6 +91,7 @@ command.party.already_in_party=<display_name><red> is already in your party.
 command.party.disband.description=Disband your current party.
 command.spy.enabled=<green>Spying is now enabled.
 command.spy.disabled=<red>Spying is now disabled.
+command.spy.description=Allows a player to view all private and channel messages they otherwise wouldn't see.
 party.player_joined=<display_name><green> joined your party.
 party.player_left=<display_name><green> left your party.
 party.cannot_use_channel=<red>You must join a party to use this channel.