[Docs] Add `default` case in Tap Dance Example 6 (#13727)
Added a `default` case in `switch(ql_tap_state.state)` at line 493 and 494.
Without it compile firmware with Example 6 code will encounter 2 errors:
`enumeration value 'TD_NONE' not handled in switch`
`enumeration value 'TD_UNKNOWN' not handled in switch`