xtimer: fix small typo

sys/xtimer/xtimer.c

@@ -118,7 +118,7 @@ void _xtimer_periodic_wakeup(uint32_t *last_wakeup, uint32_t period) {
              * to get here from the beginning of xtimer_periodic_wakeup()
              *
              * Since interrupts are normally enabled inside this function, this time may
-             * be undeterministic. */
+             * be nondeterministic. */
             target = _xtimer_now() + offset;
         }
         mutex_lock(&mutex);