contents(quiz): update retina graphics question with picturefill link

[codesandtags]

After reading the quiz question on Greatfrontend

• https://www.greatfrontend.com/questions/quiz/have-you-ever-worked-with-retina-graphics-if-so-when-and-what-techniques-did-you-use

I have noticed there was a pending reference to Picturefill polyfill, so I'm adding the reference.