[filter-effects] Fix code example

Author: alkorlos

filter-effects/Overview.bs

@@ -723,17 +723,17 @@ If a <a element>filter</a> element has no filter primitive tree then the element
 
 <div class=example>
     An example of multiple filter primitive trees:
-    <pre><code highlight=svg>
-        &lt;filter id="filter">
-          &lt-- The first filter primitive tree. Ignored for filter process. -->
-          &lt;feColorMatrix type="hueRotate" values="45"/>
-          &lt;feOffset dx="10" dy="10"/>
-          &lt;feGaussianBlur stdDeviation="3"/>
-          &lt-- The primary filter primitive tree. -->
-          &lt;feFlood flood-color="green" result="flood"/>
-          &lt;feComposite operator="in" in="SourceAlpha" in2="flood"/>
-        &lt;/filter>
-    </code></pre>
+    <xmp class=lang-markup>
+        <filter id="filter">
+          <!-- The first filter primitive tree. Ignored for filter process. -->
+          <feColorMatrix type="hueRotate" values="45"/>
+          <feOffset dx="10" dy="10"/>
+          <feGaussianBlur stdDeviation="3"/>
+          <!-- The primary filter primitive tree. -->
+          <feFlood flood-color="green" result="flood"/>
+          <feComposite operator="in" in="SourceAlpha" in2="flood"/>
+        </filter>
+   </xmp>
 
     The above filter has 2 filter primitive trees with the filter primitives:
     1. <a element>feColorMatrix</a>, <a element>feOffset</a> and <a element>feGaussianBlur</a> (with <a element>feGaussianBlur</a> being the root filter primitive of the tree) as well as