doc: fix asciidoc format compatibility in pretty-formats.adoc

doc: fix asciidoc format compatibility in pretty-formats.adoc

Asciidoc.py and Asciidoctor do not process the '+' verbatim the same way. A
span is detected when the format sign (here '+')is preceded by a non-word
character. It seems that '{nbsp}' is considered a non-word sign by
Asciidoc.py, but not by Asciidoctor.

Using a double format-sign opens 'unconstrained' span, independent on the
preceding character in both engines.

The '+' sign is used instead of the backtick '`' because it is not processed
as synopsis in asciidoc.py. Unfortunately, the post-processing of verbatim
synopsis in asciidoctor cannot be bypassed and formatting of the parentheses
is forced in syntax sign instead of keywords, unless a proper grammar
analyzer is used.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>