Guide preformatted overflow

[anddam (Andrea D'Amore)]

The Guide has a very wide size due to the presence of <pre> html element that host preformatted text, preformatted text blocks don't wrap lines, this makes horizontal scroll bar appear. I suggest to add overflow: auto; to class .programlisting in docbook.css , this won't change much the reading of preformatted text as there is only a single ​very long line but will improve the readability of the page.