Correct comments about doxygen format

chang /*@{*/ to /**@{*/ and /*@}*/ to /**@}*/

src/timer.c

@@ -63,7 +63,7 @@ static void (*rt_timer_timeout_hook)(struct rt_timer *timer);
  * @addtogroup Hook
  */
 
-/*@{*/
+/**@{*/
 
 /**
  * This function will set a hook function, which will be invoked when timer
@@ -76,7 +76,7 @@ void rt_timer_timeout_sethook(void (*hook)(struct rt_timer *timer))
     rt_timer_timeout_hook = hook;
 }
 
-/*@}*/
+/**@}*/
 #endif
 
 static void _rt_timer_init(rt_timer_t timer,
@@ -164,7 +164,7 @@ void rt_timer_dump(rt_list_t timer_heads[])
  * @addtogroup Clock
  */
 
-/*@{*/
+/**@{*/
 
 /**
  * This function will initialize a timer, normally this function is used to
@@ -710,4 +710,4 @@ void rt_system_timer_thread_init(void)
 #endif
 }
 
-/*@}*/
+/**@}*/