Fix example code in README.md
Fix example code in README.md

When `User.find` doesn't find any record,
it raises `ActiveRecord::RecordNotFound`.

vovimayhem/doorkeeper,ifeelgoods/doorkeeper,identification-io/doorkeeper,stormz/doorkeeper,telekomatrix/doorkeeper,stefania11/doorkeeper,doorkeeper-gem/doorkeeper,jasl/doorkeeper,lalithr95/doorkeeper,AdStage/doorkeeper,ukasiu/doorkeeper,ValMilkevich/doorkeeper,ifeelgoods/doorkeeper,doorkeeper-gem/doorkeeper,nbulaj/doorkeeper,coinbase/modified-doorkeeper,moneytree/doorkeeper,EasterAndJay/doorkeeper,outstand/doorkeeper,vanboom/doorkeeper,vanboom/doorkeeper,outstand/doorkeeper,6fusion/doorkeeper,coinbase/modified-doorkeeper,daande/doorkeeper,mavenlink/doorkeeper,dollarshaveclub/doorkeeper,outstand/doorkeeper,ValMilkevich/doorkeeper,shivakumaarmgs/doorkeeper,calfzhou/doorkeeper,ezilocchi/doorkeeper,jasl/doorkeeper,ministryofjustice/doorkeeper,ngpestelos/doorkeeper,kolorahl/doorkeeper,phillbaker/doorkeeper,kolorahl/doorkeeper,identification-io/doorkeeper,mavenlink/doorkeeper,stefania11/doorkeeper,Uysim/doorkeeper,shivakumaarmgs/doorkeeper,daande/doorkeeper,stormz/doorkeeper,coinbase/modified-doorkeeper,ngpestelos/doorkeeper,moneytree/doorkeeper,Uysim/doorkeeper,mavenlink/doorkeeper,EasterAndJay/doorkeeper,nbulaj/doorkeeper,AICIDNN/doorkeeper,jasl/doorkeeper,pmdeazeta/doorkeeper,ukasiu/doorkeeper,pmdeazeta/doorkeeper,vovimayhem/doorkeeper,doorkeeper-gem/doorkeeper,AdStage/doorkeeper,dk1234567/doorkeeper,dollarshaveclub/doorkeeper,AICIDNN/doorkeeper,6fusion/doorkeeper,ministryofjustice/doorkeeper,GeekOnCoffee/doorkeeper,lalithr95/doorkeeper,ezilocchi/doorkeeper,telekomatrix/doorkeeper,dk1234567/doorkeeper,GeekOnCoffee/doorkeeper,ifeelgoods/doorkeeper,calfzhou/doorkeeper