[css-variables] Fix the custom property serialization rule to match new casing decision.

Author: tabatkins

css-variables/Overview.html

@@ -732,9 +732,8 @@ <h3 id=the-cssstyledeclaration-interface><span class=secno>4.1. </span>
   <h4 id=serializing-custom-props><span class=secno>4.1.1. </span>
    Serializing Custom Properties</h4>
 
-  <p> When a custom property name is serialized, the "var-" prefix must be
-   serialized in lower-case, as a literal "var-". The rest of the property
-   name must be serialized in the original case.
+  <p> Custom property names must be serialized exactly as provided by the
+   author.
 
   <h3 id=the-cssvariablesdeclaration-interface><span class=secno>4.2. </span>
    The <a

css-variables/Overview.src.html

@@ -575,10 +575,7 @@ <h4 id='serializing-custom-props'>
 Serializing Custom Properties</h4>
 
 	<p>
-		When a custom property name is serialized,
-		the "var-" prefix must be serialized in lower-case,
-		as a literal "var-".
-		The rest of the property name must be serialized in the original case.
+		Custom property names must be serialized exactly as provided by the author.
 
 <h3 id='the-cssvariablesdeclaration-interface'>
 The <code>CSSVariablesDeclaration</code> Interface</h3>