new Buffer is deprecated

in `server/src/resolvers.js` in part 3 of the tutorial:

`if (user) return new Buffer(email).toString('base64');` should be changed to:
`if (user) return Buffer.from(email).toString('base64');`

Open server/src/resolvers.js and locate the resolver code in part 3 of the tutorial. Replace the deprecated Buffer construction with the Buffer.from form shown in the issue, then verify that the tutorial’s relevant checks still pass and that the deprecated usage is gone.

Written by the indexing model from the issue text.