Documentation: clarify quoting in "git rm" example

Documentation: clarify quoting in "git rm" example

Intended output:

	git rm Documentation/\*.txt
		Removes all *.txt files from the index that are under
		the Documentation directory and any of its
		subdirectories.

		Note that the asterisk * is quoted from the shell in
		this example; this lets git, and not the shell, expand
		the pathnames of files and subdirectories under the
		Documentation/ directory.

Without this change, there are too many backslashes output.
Tested with asciidoc 8.5.2.

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