doc: update outdated vtimer documentation

vtimer was changed in 6ee5e737f9 to use a
user specified type for vtimer_set_msg(), but documentation didn't
reflect this new API.

sys/include/vtimer.h

@@ -100,7 +100,7 @@ int vtimer_usleep(uint32_t us);
 int vtimer_sleep(timex_t time);
 
 /**
- * @brief   set a vtimer with msg event handler of type @ref MSG_TIMER
+ * @brief   set a vtimer with msg event handler of the specified type
  * @param[in]   t           pointer to preinitialised vtimer_t
  * @param[in]   interval    vtimer timex_t interval
  * @param[in]   pid         process id