Fix Resolution unit "dppx" typo

Per the Standard: http://www.w3.org/TR/css3-values/

6.4. Resolutions: the <resolution> type and ‘dpi’, ‘dpcm’, ‘dppx’ units

Resolution units are dimensions denoted by <resolution>.
The resolution unit identifiers are:

dpi: dots per inch
dpcm: dots per centimeter
dppx: dots per ‘px’ unit