poll: fix a typo in comment

Convert duplicated sys_poll to select. As Kosaki suggests, sys_poll() and
sys_select() are now hidden by SYSCALL_DEFINEx() macros so it would be
better to use plain select/poll syscall name.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>