Setting of FIREBASE_PRIVATE_KEY for vercel needs update for new vercel UI

[krthush]

Did you have difficulty using or understanding an aspect of next-firebase-auth?
Setting FIREBASE_PRIVATE_KEY env on vercel through web ui

What documentation would you like to add or change?
If possible, provide an example of the documentation you'd like to see.
Using the vercel UI now its possible add the key in just following manner (no need to single quotes):

Version
What version of next-firebase-auth are you using?
^0.13.1

[kmjennison]

Closed in #385