Documentation references to outdated ``bob.db.verification.utils``

Currently builds fail because the TrainingSet references the documentation of bob.db.verification.utils, which has gone now.

I tried to use bob.db.base instead, but I get a similar error. I assume that this is because this package does not depend on bob.db.base. We'd need to apply the same strategy as done for bob.bio.base, i.e., writing an doc/extra-intersphinx.txt and updating the doc/conf.py accordingly.

@tiago.pereira As you did that for bob.bio.base, I assign this to you.