[doc] Fix i18n.py doc
[doc] Fix i18n.py doc

PLURAL is supported by twtranslate since 2015/10/09
and twntranslate is deprecated

Change-Id: I6eca2377c560abc8cf984eb961d98aa8df7f121b

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