Updated Course End Date info

To be more specific about end dates and certificates.

https://edx-wiki.atlassian.net/secure/RapidBoard.jspa?rapidView=110&view=detail&selectedIssue=DOC-298
suggest "The course end date is the date after which students can no longer earn credit toward certificates". It's a little abrupt, but I'd like to avoid using the word "audit" since we have a very specific meaning with our Audit track.