Use headset icon for audio mode

src/invidious/views/watch.ecr

@@ -40,7 +40,7 @@
     </a>
     <% else %>
     <a title="<%=translate(locale, "Audio mode")%>" href="/watch?<%= env.params.query %>&listen=1">
-        <i class="icon ion-ios-volume-high"></i>
+        <i class="icon ion-md-headset"></i>
     </a>
     <% end %>
 </h1>