Update the location of the configuration path in the documentation #149

https://github.com/kategengler/ember-cli-code-coverage#configuration

In 0.3.12 #100 was released that changed the path of the configuration file from config/coverage.js to tests/config/coverage.js (317e7dd) and it remains this path in the current version of this addon - https://github.com/kategengler/ember-cli-code-coverage/blob/master/lib/config.js#L23 - but the documentation still references the old path of config/coverage.js and should be updated to reflect tests/config/coverage.js