docs(contributing): Fix 'addContributor' example
docs(contributing): Fix 'addContributor' example

When adding myself as a contributor I followed these instructions and
tried `yarn start addContributor dsingleton`, which threw this error:

```
Scripts must resolve to strings. There is no script that can be resolved from "dsingleton" https://github.com/kentcdodds/nps/blob/v5.8.2/other/ERRORS_AND_WARNINGS.md#missing-script
error Command failed with exit code 1.
```

I'm not super familiar with `nps`, but i'm assuming it's taking
`<YOUR_GITHUB_USERNAME>` as an additional script, rather than input to
the `addContributor` script.

You get prompted to enter your username regardless, and the docs already
say to follow the prompts.

prettier/prettier-atom