scripts/spelling.txt: add "arbitrary" typo

Add "abitrary||arbitrary".

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Link: https://lkml.kernel.org/r/6bf6520d-787d-5749-09b5-ff92185f501f@lineo.co.jp


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>