Fix mistake of custom header HTML example

lib/views/admin/customize.html

@@ -215,7 +215,7 @@
 
         <p class="help-block">
           Examples:
-          <pre><code>&lt;script src="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@9.12.0/build/languages/yaml.min.js" defer&gt;&lt;script&gt;</script></code></pre>
+          <pre><code>&lt;script src="https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@9.12.0/build/languages/yaml.min.js" defer&gt;&lt;/script&gt;</script></code></pre>
         </p>
 
         <div class="form-group">