Small bug in debug message

The debug message for "enrollment" has an error that translates to a formatting error if -vvv is used:

Traceback (most recent call last):
  File "/idiap/resource/software/bob/envs/v300py36/lib/python3.6/logging/__init__.py", line 992, in emit
    msg = self.format(record)
  File "/idiap/resource/software/bob/envs/v300py36/lib/python3.6/logging/__init__.py", line 838, in format
    return fmt.format(record)
  File "/idiap/resource/software/bob/envs/v300py36/lib/python3.6/logging/__init__.py", line 575, in format
    record.message = record.getMessage()
  File "/idiap/resource/software/bob/envs/v300py36/lib/python3.6/logging/__init__.py", line 338, in getMessage
    msg = msg % self.args
TypeError: %d format: a number is required, not str

The message should have a %s instead of %d for the model name.