scripts/spelling.txt: add "issus" typo

Add "issus" and correct it as "issues".

Link: http://lkml.kernel.org/r/20200105221950.8384-1-luca@lucaceresoli.net


Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>