[bugfix] illustratewikidata: rename page_image key in page_image_free
[bugfix] illustratewikidata: rename page_image key in page_image_free

The name of the key is no longer page_image but is now page_image_free.
So illustrate_wikidata.py is actually entirely broken.
Note: I don't think that we need, with this script, to get non-free images too. So I'll just implement the rename, not the both elements key.

Bug: T169447
Change-Id: Ia28c472c88baaad68f0042057905a0dbcd433ec8

magul/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,npdoty/pywikibot,hasteur/g13bot_tools_new,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,happy5214/pywikibot-core