doc: update markup for release note "release date" block

Bruce Momjian <bruce@momjian.us>

Commit: 9ed74fd463ede1db4ce829f9ff461d0b7f28f1f3
Author: Bruce Momjian <bruce@momjian.us>
Date: 2017-05-12T22:32:04Z
Releases: 10.0
doc:  update markup for release note "release date" block

This has to be backpatched to all supported releases so release markup
added to HEAD and copied to back branches matches the existing markup.

Reported-by: Peter Eisentraut

Discussion: 2b8a2552-fffa-f7c8-97c5-14db47a87731@2ndquadrant.com

Author: initial patch and sample markup by Peter Eisentraut

Backpatch-through: 9.2

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