add video demo

Author: xiekun

demos/firered_tts_2/index.html

@@ -133,7 +133,7 @@ <h2>FireRedTTS-2: Towards Long Conversational Speech Generation for Podcast and
                     <h2 id="app-video" style="text-align: center;">Video Demo</h2>
                     <div class="video-container">
                         <video controls>
-                            <source src="video/blog_chat.mp4" type="video/mp4">
+                            <source src="https://github.com/FireRedTeam/fireredteam.github.io/blob/main/demos/firered_tts_2/video/blog_chat.mp4" type="video/mp4">
                         </video>
                     </div>
                 </div>