Add missing bracket for Tap-Hold code sample (#9675)

The `TAPPING_TERM_PER_KEY` code sample is missing a closing bracket.
This adds it.

docs/tap_hold.md

@@ -33,6 +33,7 @@ uint16_t get_tapping_term(uint16_t keycode, keyrecord_t *record) {
         default:
             return TAPPING_TERM;
     }
+}
 ```