git-add documentation: Fix shell quoting example

git-add documentation: Fix shell quoting example

When 921177f (Documentation: improve "add", "pull" and "format-patch"
examples, 2008-05-07) converted this from enumeration header to displayed
text, it failed to adjust for the AsciiDoc's rule to quote backslashes.
In displayed text, backslash is shown verbatim, while in enumeration
header, we need to double it.

We have a similar construct in git-rm.txt documentation, and need to be
careful when somebody wants to update it to match the style of the "git
add" example.

Noticed by: Greg Bacon <gbacon@dbresearch.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>