[css-values-4] Per WG resolution, dppx is the canonical <resolution> unit. Fixes w3c#2241.

Author: tabatkins

css-values-4/Overview.bs

@@ -1434,7 +1434,7 @@ Resolution Units: the <<resolution>> type and ''&lt;resolution&gt;/dpi'', ''&lt;
 	or the 'image-resolution' property defined in [[CSS3-IMAGES]].
 
 	All <<resolution>> units are <a>compatible</a>,
-	and ''&lt;resolution&gt;/x'' is their <a>canonical unit</a>.
+	and ''&lt;resolution>/dppx'' is their <a>canonical unit</a>.
 
 	<p class="note">Note that due to the 1:96 fixed ratio of CSS ''in'' to CSS ''px'',
 	''1dppx'' is equivalent to ''96dpi''.