Documentation: clarify quoting in "git add" example

Documentation: clarify quoting in "git add" example

The intended text looks like this:

	· Adds content from all *.txt files under Documentation
	  directory and its subdirectories:

		$ git add Documentation/\*.txt

	  Note that the asterisk * is quoted from the shell in this
	  example; this lets the command include the files from
	  subdirectories of Documentation/ directory.

The current asciidoc 8.5.2 output has a backslash before _every_
asterisk, which is more confusing than it needs to be.

Reported-by: Frédéric Brière <fbriere@fbriere.net>
Cc: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>