Documentation: remove unnecessary backslashes

Documentation: remove unnecessary backslashes

asciidoctor does not remove backslashes used to escape curly brackets from
the HTML output if the contents of the curly brackets are empty or contain
at least a <, -, or space.  asciidoc does not require the backslashes in
these cases, so just remove them.

Signed-off-by: Matthew Kraai <matt.kraai@abbott.com>
Reported-by: Philip Oakley <philipoakley@iee.org>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>