(bug) Instruction errors in CONTRIBUTING.md

CONTRIBUTING instructions include commands `npm run test` and `npm run test:watch`; these generate errors:

```
npm ERR! Missing script: "test"
npm ERR! 
npm ERR! To see a list of scripts, run:
npm ERR!   npm run
...
```

## Steps to reproduce

Run either command `npm run test` or `npm run test:watch`.
