Fix incorrect template var

Author: jc21

src/backend/templates/_listen.conf

@@ -1,5 +1,5 @@
   listen 80;
 {% if certificate -%}
-  listen 443 ssl{% if http2 %} http2{% endif %};
+  listen 443 ssl{% if http2_support %} http2{% endif %};
 {% endif %}
   server_name {{ domain_names | join: " " }};