Got rid of use of undefined variables

docs/guides/voice/samples/joining_audio.cs

@@ -3,8 +3,8 @@
 public async Task JoinChannel(IVoiceChannel channel = null)
 {
     // Get the audio channel
-    channel = channel ?? (msg.Author as IGuildUser)?.VoiceChannel;
-    if (channel == null) { await msg.Channel.SendMessageAsync("User must be in a voice channel, or a voice channel must be passed as an argument."); return; }
+    channel = channel ?? (Context.User as IGuildUser)?.VoiceChannel;
+    if (channel == null) { await Context.Channel.SendMessageAsync("User must be in a voice channel, or a voice channel must be passed as an argument."); return; }
 
     // For the next step with transmitting audio, you would want to pass this Audio Client in to a service.
     var audioClient = await channel.ConnectAsync();