Minor update to the newly added XML schema file

[larsbarring]

There is a schema mismatch between the new XML element for the "first published date" in the XML file. The element was introduced in cf-conventions/#511 as first_published, but was in the schema file written as first_published_date. The "_date" part is quite unnecessary and even not correct as the information carried by this element is in fact an xml dateTime object.

See PR #482

[larsbarring]

Hi Sadie @sadielbartholomew: May I one last time ask you: this time to merge this PR #482, which is correcting a minor oversight on my part. After closing this, and #516 everything the set to dig into #470 that will keep me busy for a while :-))

Many thanks,
Lars

[sadielbartholomew]

Good spot Lars. Sure, I will review it and merge if good now.