Update demo.html

Author: anurag01a

src/cloudflare-worker/demo/demo.html

@@ -30,7 +30,7 @@ <h1>LeetCode Stats Card</h1>
         <div>
             <button onclick="preview()">Preview</button>
             <button onclick="go()">Go</button>
-            <button onclick="md()">Markdwon</button>
+            <button onclick="md()">Markdown</button>
         </div>
         <div><img id="preview" /></div>
         <div style="margin-top: 20px">