Update recording_with_microphone.rst (#5986)

* Update recording_with_microphone.rst

Added the detail that you need to enable the mic in the project settings, as I just spent about two hours wondering why it wasn't working like the example.

tutorials/audio/recording_with_microphone.rst

@@ -10,6 +10,8 @@ A simple demo is included in the official demo projects and will be used as
 support for this tutorial:
 `<https://github.com/godotengine/godot-demo-projects/tree/master/audio/mic_record>`_.
 
+You will need to enable audio input in the project settings, or you'll just get empty audio files.
+
 The structure of the demo
 -------------------------