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

Bruce Momjian <bruce@momjian.us>

Commit: b5fc98aedf513780b9b61c904cfcb87a7d02fbf1
Author: Bruce Momjian <bruce@momjian.us>
Date: 2017-05-12T22:31:54Z
Releases: 9.2.22
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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