Add proper response type for GuildBans docs

The previous documentation stated that the response type was an array of User structures, when the function return an array of GuildBan structures.

restapi.go

@@ -675,7 +675,7 @@ func (s *Session) GuildLeave(guildID string) (err error) {
 	return
 }
 
-// GuildBans returns an array of User structures for all bans of a
+// GuildBans returns an array of GuildBan structures for all bans of a
 // given guild.
 // guildID   : The ID of a Guild.
 func (s *Session) GuildBans(guildID string) (st []*GuildBan, err error) {