Minor update for mock_open docs
Minor update for mock_open docs

nett55/mock,frankier/mock,derwiki-adroll/mock,rbtcollins/mock,OddBloke/mock,lord63-forks/mock,johndeng/mock,sorenh/mock,testing-cabal/mock,dannybtran/mock,pypingou/mock,kostyll/mock,scorphus/mock,fladi/mock