finatra: Fix broken mixin link in testing documentation
finatra: Fix broken mixin link in testing documentation

Summary: Problem

Seen here: https://twitter.github.io/finatra/user-guide/testing/index.html,
on almost bottom page, on section

```
To make use of another ScalaTest test style, such as FunSpec or
others, see Test Mixins.
```

There's link to `Test Mixins`, but the link is broken. It's because
of missing `s` on the `html` link.

Solution

Correct the broken link.

Result

Better documentation.

Signed-off-by: Christopher Coco <ccoco@twitter.com>

TBR=true

Differential Revision: https://phabricator.twitter.biz/D123762

twitter/finatra,twitter/finatra,twitter/finatra