#3867 docs: update the example in the readme so that it uses const instead of var

[mwanago]

Closes #3867

[wesleytodd]

I just want to say that I am lukewarm on these kind of updates. I feel like it is fine and well to follow common developer practices with the examples, but I don't see a lot of value in changing perfectly functional examples.

This is coming from someone who does define my imports as consts, so I am not being grumpy about change. Just saying that making change just to do this kind of stuff is more noise than signal.

[cujarrett]

Looks good 😃

[dougwilson]

@wesleytodd I don't really care for these either, but since it does work on our target Node.js versions and perhaps will stop this particular PR in the future, I may as well merge it in.