[cssom] Fix ElementCSSInlineStyle section. (#4179)

It's a mixin now.

Fixes #4174

Author: emilio

cssom-1/Overview.bs

@@ -2782,10 +2782,10 @@ DOM Access to CSS Declaration Blocks {#dom-access-to-css-declaration-blocks}
 ============================================================================
 
 
-The {{ElementCSSInlineStyle}} Interface {#the-elementcssinlinestyle-interface}
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+<h3 id="the-elementcssinlinestyle-mixin" oldids="the-elementcssinlinestyle-interface">
+The {{ElementCSSInlineStyle}} Mixin</h3>
 
-The <code>ElementCSSInlineStyle</code> interface provides access to inline style properties of an element.
+The <code>ElementCSSInlineStyle</code> mixin provides access to inline style properties of an element.
 
 <pre class=idl>
 interface mixin ElementCSSInlineStyle {