channel id is optional for replies (#14)

yet another 1 character pull request 🙂

https://discord.com/developers/docs/resources/channel#message-reference-object

![image](https://cdn.discordapp.com/attachments/796997555752796184/889608085579112519/unknown.png)

Author: Commandtechno

src/types.ts

@@ -396,7 +396,7 @@ export namespace RequestTypes {
     files?: Array<File>,
     hasSpoiler?: boolean,
     messageReference?: {
-      channelId: string,
+      channelId?: string,
       failIfNotExists?: boolean,
       guildId?: string,
       messageId: string,