Fix a comment
Fix a comment

The comment became imprecise when one of the functions started accepting
a digest.Digest instead of a string.

(This is unrelated to the rest of the PR.)

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

masters-of-cats/image,containers/image,containers/image