update-book2: work around a regression in AsciiDoctor v2.0.19

In #1879 a regression was
reported in the way the explanation of the refspec format is rendered;
It is now broken.

The reason is a regression in AsciiDoctor (reported at
asciidoctor/asciidoctor#4624).

As a work-around, replace the `+` instances (which are mis-interpreted
if more than one exists in the same line) by `{plus}` (which AsciiDoctor
renders exactly like AsciiDoc would).

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

script/update-book2.rb

@@ -74,6 +74,9 @@ def genbook(language_code, &get_content)
   content = expand(initial_content, "progit.asc") { |filename| get_content.call(filename) }
   # revert internal links decorations for ebooks
   content.gsub!(/<<.*?\#(.*?)>>/, "<<\\1>>")
+  # work around AsciiDoctor v2.0.19 and later mis-interpreting multiple `+` as
+  # indicating the start or the end of literal monospace.
+  content.gsub!(/`\+`/, "`{plus}`")
 
   asciidoc = Asciidoctor::Document.new(content, attributes: { "lang" => language_code })
   html = asciidoc.render