Update README on the location of the coverage report.

The coverage report is in:

`target/scala-<scala-version>/scoverage-report`

And not in:

`target/scoverage-report` as the README states:

> Coverage reports will be in target/scoverage-report. There are HTML and XML reports. The XML is useful if you need to programatically use the results, or if you're writing a tool.
