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>