bugfix: rename "fetchGuildChannel" method of DiscordGuild to specify it is asynchronous

This is a breaking change. If you want to avoid the breaking change, something like this snippet could be added: ```luau function DiscordGuild.Prototype.fetchGuildChannelsAsync(self: DiscordGuild) return self:fetchGuildChannelsAsync() end ``` No idea if you want to change the method name or not, I'll leave this pull request just in case.