Fix up albumartist_website plugin debug message

Traceback (most recent call last):
  File "picard\webservice.pyo", line 285, in _process_reply
  File "C:\Program Files (x86)\MusicBrainz Picard\plugins\albumartist_website.py", line 95, in website_process
    log.debug("%s: %r: Artist Official Homepages = %s", PLUGIN_NAME, artistId, url)
NameError: global name 'url' is not defined