Documentation: AsciiDoc spells em-dash as double-dashes, not triple

Documentation: AsciiDoc spells em-dash as double-dashes, not triple

Again, we do not usually process release notes with AsciiDoc, but it
is better to be consistent.

This incidentally reveals breakages left by an ancient 5e00439f
(Documentation: build html for all files in technical and howto,
2012-10-23).  The index-format documentation was originally written
to be read as straight text without formatting and when the commit
forced everything in Documentation/ to go through AsciiDoc, it did
not do any adjustment--hence the double-dashes will be seen in the
resulting text that is rendered as preformatted fixed-width without
converted into em-dashes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>