avro

AVRO-XXXX: Correct documentation links (#845)

Links do not work in Chrome or Firefox when visiting docs at `https://github.com/apache/avro/blob/master/lang/js/doc/Home.md`.

Note the reason I was viewing the Home.md file was that the [avro-js package](https://www.npmjs.com/package/avro-js) says that for documentation:

> See doc/ folder.

It does not specifically say which doc/ folder or link to it, so I assume that it means `https://github.com/apache/avro/blob/master/lang/js/doc/`. Perhaps it means something else, where the `Api` link actually works, but I couldn't locate it. Unusual to have to search so hard to find docs.