Documentation link typos

In the **[Connecting to the Document Layer](https://foundationdb.github.io/fdb-document-layer/getting-started-mac.html#connecting-to-the-document-layer)** section, the extension for the _configuration file_ is `.md` instead of `.html`.

Same goes for the [Getting Started on Linux](https://foundationdb.github.io/fdb-document-layer/configuration.md) one.