Update recording_with_microphone.rst missing 'var'

[toivocat]

Changed "effect = AudioServer.get_bus_effect(idx, 0)" to "var effect = AudioServer.get_bus_effect(idx, 0)" so it will work.

[Calinou]

For future reference, this change isn't needed as effect is declared as a member variable above. If anything, it will cause an error where there was none previously (as GDScript doesn't allow redeclaring variables).