genbook2: simplify html fixup

We can handle "gt" and "lt" in a single regex. Not a big
deal for this simple case, but as we add more cases, it will
be nice not to have to repeat ourselves.

lib/tasks/book2.rake

@@ -112,8 +112,7 @@ task :genbook2 => :environment do
             end
           end
 
-          html.gsub!(">", ">")
-          html.gsub!("<", "<")
+          html.gsub!(%r{&([gl]t);}, '&\1;')
 
           if subsec = html.scan(/<h3>(.*?)<\/h3>/)
             subsec.each do |sub|