Fix typo in validator.py that produced imprecise output message

[awoods]

Prior to this change, running the validator on OCFL/fixtures#66 produced:

[E093] OCFL Object root inventory fixity block for digest algorithm ??? has digest..

After this change, the output is now:

[E093] OCFL Object root inventory fixity block for digest algorithm md5 has digest..

[coveralls]

Coverage increased (+0.006%) to 94.875% when pulling b5632d4 on awoods:fixity-check-typo into 2f72087 on zimeon:main.

[awoods]

Also resolves: #38

[zimeon]

Fixed in develop:

(py38) simeon@RottenApple ocfl-py> ./ocfl-validate.py fixtures/1.0/bad-objects/E093_fixity_digest_mismatch/
[E093] OCFL Object root inventory fixity block for digest algorithm ??? has digest 9eacfb9289073dd9c9a8c4cdf820ac71 for file v1/content/test.txt which doesn't match calculated digest eb1a3227cdc3fedbaec2fe38bf6c044a for that file (see https://ocfl.io/1.0/spec/#E093)
INFO:ocfl.object:OCFL object at fixtures/1.0/bad-objects/E093_fixity_digest_mismatch/ is INVALID
(py38) simeon@RottenApple ocfl-py> git pull
remote: Enumerating objects: 8, done.
remote: Counting objects: 100% (8/8), done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 8 (delta 3), reused 4 (delta 3), pack-reused 0
Unpacking objects: 100% (8/8), done.
From github.com:zimeon/ocfl-py
   a7e236f..e6e6fdc  develop    -> origin/develop
Updating a7e236f..e6e6fdc
Fast-forward
 ocfl/validator.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
(py38) simeon@RottenApple ocfl-py> ./ocfl-validate.py fixtures/1.0/bad-objects/E093_fixity_digest_mismatch/
[E093] OCFL Object root inventory fixity block for digest algorithm md5 has digest 9eacfb9289073dd9c9a8c4cdf820ac71 for file v1/content/test.txt which doesn't match calculated digest eb1a3227cdc3fedbaec2fe38bf6c044a for that file (see https://ocfl.io/1.0/spec/#E093)
INFO:ocfl.object:OCFL object at fixtures/1.0/bad-objects/E093_fixity_digest_mismatch/ is INVALID

[zimeon]

Thanks @awoods !