applications: nrf_desktop: Update source file for USB event in docs

Change aligns documentation with implementation.

Jira: NCSDK-35005

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

Author: MarekPieta

applications/nrf_desktop/api.rst

@@ -29,6 +29,6 @@ USB events
 **********
 
 | Header file: :file:`applications/nrf_desktop/src/events/usb_event.h`
-| Source file: :file:`applications/nrf_desktop/src/modules/usb_state.c`
+| Source file: :file:`applications/nrf_desktop/src/events/usb_event.c`
 
 .. doxygengroup:: nrf_desktop_usb_event