fix no element error

tools/chromium_extension/js/content-script.js

@@ -2,6 +2,7 @@
 
     let URLObj = new URL(location.href)
     console.log(URLObj)
+    if( document.querySelector("#readme table tbody")) {
 
         let audio_player = new Audio()
         audio_player.setAttribute('autoplay', 'true')
@@ -28,4 +29,5 @@
                 audio_player.setAttribute('src', audio_url);
             }
         })
+    }
 }