git-add documentation: Fix shell quoting example

When 921177f5 (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: NJunio C Hamano <gitster@pobox.com>

Documentation/git-add.txt

@@ -103,7 +103,7 @@ EXAMPLES
 and its subdirectories:
 +
 ------------
-$ git add Documentation/\\*.txt
+$ git add Documentation/\*.txt
 ------------
 +
 Note that the asterisk `\*` is quoted from the shell in this