Fix AsciiDoc errors in merge documentation

Fix AsciiDoc errors in merge documentation

In the section on conflict markers, the "<<<<<<<" sequence is compiled by
AsciiDoc into invalid XML. A way to resolve this is by inserting something
between the last two characters in that sequence (i.e. between '<' and '"').

This patch encloses the conflict markers in backticks, which renders them
in a monospace font (in the HTML version; the manual page is unaffected),
and with the pleasant side-effect that it also fixes the AsciiDoc compile
problem.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>