Fix header button for wagtail<2.10

wagtailvideos/templates/wagtailvideos/videos/index.html

@@ -24,7 +24,7 @@
 
     {% trans "Add a video" as add_video_str %}
     {% url "wagtailvideos:add_multiple" as add_link %}
-    {% include "wagtailadmin/shared/header.html" with title=video_str action_url=add_link icon="media" action_text=add_video_str search_url="wagtailvideos:index" %}
+    {% include "wagtailadmin/shared/header.html" with title=video_str add_link="wagtailvideos:add_multiple" action_url=add_link icon="media" add_text=add_video_str action_text=add_video_str search_url="wagtailvideos:index" %}
 
     <div class="nice-padding">
         {% if collections %}