writeback: fix global_dirty_limits comment runtime -> real-time

Change runtime with real-time

Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

mm/page-writeback.c

@@ -404,7 +404,7 @@ unsigned long determine_dirtyable_memory(void)
  * - vm.dirty_background_ratio  or  vm.dirty_background_bytes
  * - vm.dirty_ratio             or  vm.dirty_bytes
  * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * runtime tasks.
+ * real-time tasks.
  */
 void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
 {