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

Bruce Momjian <bruce@momjian.us>

Commit: a9787e737dc8fa8779263c4112c25b7f0792556c
Author: Bruce Momjian <bruce@momjian.us>
Date: 2017-05-12T22:31:59Z
Releases: 9.4.13
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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