fix: HTML validation issue (#2843)

files/en-us/web/css/css_houdini/index.html

@@ -32,7 +32,7 @@ <h2 id="Basic_example">Basic example</h2>
 <p>Using {{cssxref("@property")}} however, we can declare the custom property with a {{CSSxRef("@property/syntax","syntax")}} of <code>&lt;color&gt;</code>. This shows that we need this property to have a value which is a valid color.</p>
 
 <pre class="brush:css">@property --background-color {
-  syntax: '<color>';
+  syntax: '&lt;color&gt;';
   inherits: false;
   initial-value: blue;
 }</pre>