Use c8 for code coverage examples instead of nyc

[bompus]

For AVA 4, I think it would be a good time to start recommending using c8 for coverage instead of nyc.

There are only a few instances in the codebase where it is mentioned: https://github.com/avajs/ava/search?q=nyc

[novemberborn]

Yea happy to take a PR that removes all mention of nyc. I use c8 everywhere, it's not an AVA 4 thing.