docbook: fix signal_pending() argument

Since signal_pending() takes a task_struct pointer as an argument, update
the example to pass in 'current'.

Signed-off-by: Shawn Bohrer <shawn.bohrer@gmail.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>