kernel/timer.c: fix kernel-doc function parameter warning

Fix kernel-doc warning, add @timer description:

  Warning(kernel/timer.c:335): No description found for parameter 'timer'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/timer.c

@@ -326,6 +326,7 @@ EXPORT_SYMBOL_GPL(round_jiffies_up_relative);
 
 /**
  * set_timer_slack - set the allowed slack for a timer
+ * @timer: the timer to be modified
  * @slack_hz: the amount of time (in jiffies) allowed for rounding
  *
  * Set the amount of time, in jiffies, that a certain timer has