Update audio_ffmpeg.cs

AudioApplication is a required argument

docs/guides/samples/audio_ffmpeg.cs

@@ -3,7 +3,7 @@ private async Task SendAsync(IAudioClient client, string path)
     // Create FFmpeg using the previous example
     var ffmpeg = CreateStream(path);
     var output = ffmpeg.StandardOutput.BaseStream;
-    var discord = client.CreatePCMStream(1920);
+    var discord = client.CreatePCMStream(AudioApplication.Mixed, 1920);
     await output.CopyToAsync(discord);
     await discord.FlushAsync();
-}
+}