Correct typos in DefaultConfigTest.php

@vegantriathlete, Thanks for the patch corrections, i checked by applying patch manually in module file.

Neither of these grammatical errors could be caught by any automated test, so it makes sense to just fix them.

The first docblock raises some questions and has an untracked @todo, but that's out of scope here.

Committed to 8.4.x. As a documentation improvement, this issue is also RC eligible, so I've backported it to 8.3.x. Thanks @vegantriathlete!

Thanks @dhwani.addweb for reviewing. We generally do not need to confirm that the patch applies. Our automated infrastructure confirms that for us. For documentation improvements, we value your help evaluating the documentation and deciding how it can be better. In the case of this issue, I confirmed that the two grammar fixes were correct. When you review a patch, describe what you thought about and the questions you asked yourself. This is helpful for other reviewers to see what's been considered.