proofreadpage_tests: fix TestProofreadPageIndexProperty
proofreadpage_tests: fix TestProofreadPageIndexProperty

Fix en.wikisource.org test page names.

Bug: T181863
Change-Id: I00e8a94d9337458469e6c44efb777c32844b49c0

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