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

Bruce Momjian <bruce@momjian.us>

Commit: 610926e1d98121b7f9d3c175d5f8b2b75898b046
Author: Bruce Momjian <bruce@momjian.us>
Date: 2017-05-12T22:32:03Z
Releases: 9.6.4
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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