Minor edit

src/Discord.Net/Models/Message.cs

@@ -341,7 +341,7 @@ namespace Discord
         public Task Acknowledge()
         {
             if (Client.Config.Mode != DiscordMode.Client)
-                throw new InvalidOperationException("Message.Acknowledge may only be used in Client mode.");
+                throw new InvalidOperationException("This function may only be used in Client mode.");
 
             if (User.Id != Client.CurrentUser.Id)
                 return Client.ClientAPI.Send(new AckMessageRequest(Channel.Id, Id));