smp: fix documentation in include/linux/smp.h

smp: Fix documentation.

Fix documentation in include/linux/smp.h: smp_processor_id()

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>