[cssom] Change serialization of <resolution> to use dppx

WG resolution at
https://lists.w3.org/Archives/Public/www-style/2016Jun/0118.html

Fixes w3c#178.

Author: zcorpan

cssom/Overview.bs

@@ -2439,8 +2439,8 @@ depends on the component, as follows:
  by the literal string "<code>%</code>" (U+0025).
 
  <dt>&lt;resolution>
- <dd>The resolution in dots per centimeter serialized as per
- &lt;number> followed by the literal string "<code>dpcm</code>".
+ <dd>The resolution in dots per CSS pixel serialized as per
+ &lt;number> followed by the literal string "<code>dppx</code>".
 
  <dt>&lt;shape>
  <dd>