Fix expression pedal pin: 5 -> 4

Constructor was still GPIO5 despite physical move to GPIO4.
The pedal was reading the wrong (unconnected) pin, so no ADC changes.

src/main.cpp

@@ -19,7 +19,7 @@ DefaultSwitchStub switch_driver;
 UsbMidiTransport midi_transport;
 
 AppTask controller(&led_driver, &switch_driver, &midi_transport);
-ExpressionPedal exp_pedal(5);
+ExpressionPedal exp_pedal(4);
 
 TaskHandle_t midi_task_handle = NULL;