Fixed a bad documentation comment for State.ChannelAdd (#338)

state.go

@@ -295,7 +295,7 @@ func (s *State) Role(guildID, roleID string) (*Role, error) {
 	return nil, errors.New("role not found")
 }
 
-// ChannelAdd adds a guild to the current world state, or
+// ChannelAdd adds a channel to the current world state, or
 // updates it if it already exists.
 // Channels may exist either as PrivateChannels or inside
 // a guild.