[css-masking-1][editorial] fix formatting

Author: svgeesus

css-masking-1/Overview.bs

@@ -1261,7 +1261,7 @@ The 'mask-type' property allows the author of the <a element>mask</a> element to
 
     In the next example the computed value of 'mask-mode' is ''mask-mode/alpha'' and overrides the preference on the <a element>mask</a> element that is computed to ''mask-type/luminance''. The <a>mask layer image</a> is used as an alpha mask.
 
-    <pre><code highlight=html>
+    <pre><code highlight=svg>
         &lt;svg>
          &lt;mask style="mask-type: luminance;" id="mask2">
            ...