Fix test_WbMonolingualText_invalid_text
Fix test_WbMonolingualText_invalid_text

Apparently the error message has changed, update it.

Bug: T216000
Change-Id: I30fb84744cf34d6a57bb13fa2a62b7c27382e29a

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core