[css-print] Selector section references lang attribute that was s uperceded by xml:lang

Section 3. Selectors contains a reference to lang attribute in the section
on attribute selectors, 'Matches any E element whose "lang" attribute value
has a hyphen-separated list of values beginning (from the left) with "en"' 

The attribute has been replaced in XHTML and XHTML-Print by the xml:lang
attribute.

Jim