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>
1 file changed