Fix broken link in contributing doc.

[Awjin]

Fixes #1017

[albert-github]

In my opinion, this correction is incorrect it will give:

 It's defined using Dart's [JS interop package][],

the correction should be a few lines up, giving the entire, corrected paragraph:

### Changing the Node API

Most of Dart Sass's code is shared between Dart and Node.js, but the API that's
exported by the [`sass`][npm] npm package is Node-specific. It's defined using
Dart's [JS interop package][JS], and it's tested by compiling the Dart package to
JS and loading that JS using JS interop to best simulate the conditions under
which it will be used in the real world.

[npm]: https://www.npmjs.com/package/sass
[JS]: https://pub.dartlang.org/packages/js

so the corrected lines are:

Dart's [JS interop package][JS], and it's tested by compiling the Dart package to

and

[JS]: https://pub.dartlang.org/packages/js

[jathak]

Reference links with no label in the second set of brackets are implicitly labeled with the text being linked, so JS interop package correctly refers to that link. See https://daringfireball.net/projects/markdown/syntax#link

[albert-github]

One is never to old to learn... , though I would prefer the explicit version (but that is my personal opinion)..