Fix reference to healthCheckRegistry

This just fixes typos in the documentation. 

There is another issue though:

The `Checked` trait defines `healthCheckRegistry` where it should be saying just `registry` as [CheckedBuilder#L32](https://github.com/erikvanoosten/metrics-scala/blob/master/src/main/scala/nl/grons/metrics/scala/CheckedBuilder.scala#L32) says so. But I think I would actually prefer the `CheckedBuilder` to define `healthCheckRegistry` instead of just `registry`.