ipc/sem.c: spelling fix

s/purpuse/purpose/

Link: https://lkml.kernel.org/r/20210319221432.26631-1-unixbhaskar@gmail.com


Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>