Code Review: Fix broken link to coverage report

The link to the coverage report in the pdl github.io [Java API](https://usgs.github.io/pdl/userguide/java_api.html) page is broken, please fix.

Also, consider moving this coverage report to codecov or codacy.  If codecov or codacy is used, please add a link / badge to the main page, or otherwise make it more visible. 