README: Fix 'mutt-ldap.py' -> 'mutt_ldap.py' typos

These are out of date after:

  commit d3879f6e4509c35f0bcb79e0e4eb99a1d767fd35
  Author: W. Trevor King <wking@tremily.us>
  Date:   Sun Jan 20 14:13:42 2013 -0500

    mutt_ldap.py: Rename with underscore for package import