Fix a typo

An argument group was initialized with a "desription" instead of
a "description".
Minor help text clean-up

This cleans up a comma-separated list in a help text so that each
element is individually quoted.
Handle no specified audits

The cognosco.reports._make_repos() helper function failed to take into
account the possibility of no audits being specified on the command
line.  This adds the necessary code to tolerate this condition.