[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`

docs/feature_tap_dance.md

@@ -490,6 +490,8 @@ void ql_finished(qk_tap_dance_state_t *state, void *user_data) {
                 layer_on(_MY_LAYER);
             }
             break;
+        default:
+            break;
     }
 }