Documentation: fix misrender of pretty-formats in Asciidoctor

Neither the AsciiDoc nor the Asciidoctor documentation specify whether
the same number of delimiter characters must be used to end a block as
to begin it, although both sets of documentation show exactly matching
pairs.  AsciiDoc allows mismatches, but AsciiDoctor apparently does not.
Adjust the pretty formats documentation to use matching pairs to prevent
a misrendering where the remainder of the document was rendered as a
listing block.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'bc/asciidoc-pretty-formats-fix' into jch

* bc/asciidoc-pretty-formats-fix:
  Documentation: fix misrender of pretty-formats in Asciidoctor