Client is global variable (#789)

In the previous section of the tutorial https://github.com/RogueException/Discord.Net/blob/dev/docs/guides/getting_started/samples/intro/client.cs, the client it a global variable with an underscore

docs/guides/getting_started/samples/intro/message.cs

@@ -1,7 +1,7 @@
 public async Task MainAsync()
 {
 	// client.Log ...
-	client.MessageReceived += MessageReceived;
+	_client.MessageReceived += MessageReceived;
 	// ...
 }
 
@@ -11,4 +11,4 @@ private async Task MessageReceived(SocketMessage message)
 	{
 		await message.Channel.SendMessageAsync("Pong!");
 	}
-}
+}