gitmailmap.txt: fix rendering of e-mail addresses

gitmailmap.txt: fix rendering of e-mail addresses

Both AsciiDoc and Asciidoctor are eager to pick up the e-mail addresses
in this document and turn them into references at the bottom of the
manpage / clickable links. We don't really need that for these dummy
addresses. Spell "@" as "@" to make them not do this. In the open
block, we can instead avoid this by indenting the contents, similar to
the earlier blocks.

Fix a backtick which should have been a single quote mark. With all the
quoting that is going on around here, this mistake trips up the parsing
and rendering quite a bit.

Before this commit, we have the same failure mode with AsciiDoc 8.6.10
and Asciidoctor 1.5.5, and this change makes both of them happy.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>