Update DMChannel, GroupDMChannel for docs (#1038)

src/structures/GroupDMChannel.js

@@ -121,8 +121,8 @@ class GroupDMChannel extends Channel {
   }
 
   // These are here only for documentation purposes - they are implemented by TextBasedChannel
+  send() { return; }
   sendMessage() { return; }
-  sendTTSMessage() { return; }
   sendEmbed() { return; }
   sendFile() { return; }
   sendCode() { return; }