fix wording

addresses: RIOT-OS#1041 (comment)

Author: LudwigKnuepfer

core/include/hwtimer.h

@@ -119,7 +119,7 @@ void hwtimer_wait(unsigned long ticks);
 
 /**
  * @brief   determine if the hwtimer module is initialized
- * @return  true when the hwtimer module is initialized
+ * @return  true if the hwtimer module is initialized
  */
 int hwtimer_active(void);