30-seconds-of-code: Update explanation for mapObject

[Chalarangelo]

mapObject's explanation is not up to standards with our current format. It should be updated to match the rest of the snippets.

We could also make the example a one-liner.

[ezracelli]

Perhaps a simpler implementation:

const mapObject = (arr, fn) => Object.assign(...arr.map(el => ({ [el]: fn(el) })))

[icncsx]

@ezracelli do you want to make a PR?