lint: use TextChannel to get GuildID

src/Discord.Net.Core/Extensions/MessageExtensions.cs

@@ -5,7 +5,7 @@ namespace Discord
         public static string GetJumpUrl(this IMessage msg)
         {
             var channel = msg.Channel;
-            return $"https://discordapp.com/channels/{(channel is IDMChannel ? "@me" : $"{(channel as IGuildChannel).GuildId}")}/{channel.Id}/{msg.Id}";
+            return $"https://discordapp.com/channels/{(channel is IDMChannel ? "@me" : $"{(channel as ITextChannel).GuildId}")}/{channel.Id}/{msg.Id}";
         }
     }
 }