enable another markdown lint rule (#1142)
enable another markdown lint rule (#1142)

* enable another markdown lint rule

We ran into this in a PR review so I think it is reasonable to enable
it.

As an aside, I researched if markdown CLI let's you enforce reference style links. By default no.
But the underlying library does let you specify custom rules.
See another project's [implementation](https://github.com/webhintio/hint/commit/c412f9aa7ba99eb7ef6c20b7c496d629530f3ecf).

In our case, I find that too much trouble and maintenance for now.

* enable correct value!
exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust