Update tutorial-hello-world.md

[moodmosaic]

Fix a small grammar typo

[unclemantis]

How about killing the Mocha testing method and replacing with Clarinet by HiroSystems!

[moodmosaic]

That would be the best as clarinet test is the de facto now.

FWIW I wrote some articles covering some of the clarinet test internals and there's more to come.

[moodmosaic]

(In the meantime, I think nothing stops this from being merged.)