fix(katex): resolved removed class from demo during merging

demos/sequence.html

@@ -143,7 +143,7 @@
         </pre
     >
     <hr />
-    <pre>
+    <pre class="mermaid">
       sequenceDiagram
       participant Alice
       participant Bob
@@ -196,7 +196,7 @@
       2->>3: $$\frac{\text{d}}{\text{d}x}{3x^2+2x+1}$$
       3-->>2: $$6x+2$$
     </pre>
-    <hr />
+
     <script type="module">
       import mermaid from './mermaid.esm.mjs';
       mermaid.initialize({