Revise mentions of Istanbul as it's now deprecated

[iteles]

In light of dwyl/learn-istanbul#20 we should revise mentions/usage of istanbul in this tutorial in favour of nyc.

[nelsonic]

@iteles do you want to pair on this revision/update
or would you favour a Pull Request from someone who has read the tutorial and found it useful?
My guess is that the people reading this tutorial probably won't be reading the issues.
So we are unlikely to get a random PR from a reader. 💭

[iteles]

Seems like a good short-ish project for this or next weekend 👍
It's not super high priority though.

[vedant-z]

@nelsonic I am willing to make a PR for this. I just need little bit of help on what needs to e done and how should I proceed.