Fix a bug: Liquid does not allow parenthesis for logical operations.

Author: aki-nishimura

_layouts/talk.html

@@ -4,7 +4,7 @@
 
 {% include base_path %}
 
-{% if (page.header.overlay_color or page.header.overlay_image) or page.header.image %}
+{% if page.header.overlay_color or page.header.overlay_image or page.header.image %}
   {% include page__hero.html %}
 {% endif %}