Update README
Update README

While reading the documentation for Passport I noticed the word 'flexibility' was spelled incorrectly on line 15. This pull request fixes the spelling error.
arover/passport,dentrinh/passport,jameswomack/passport-restify,jaredhanson/passport,bartuspan/passport,imjerrybao/passport,Trult/passport,mcanthony/passport,kolomiichenko/passport,DaAwesomeP/passport,shubham13jain/passport