efi/libstub: Correct comment typos

Fix a couple typos in comments.

Signed-off-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/r/ec53e67b3ac928922807db3cb1585e911971dadc.1588273612.git.joe@perches.com


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>