refactor: fix reference to comment parameter `site.comments.provider` (#1629)

_includes/comments.html

@@ -1,5 +1,5 @@
-<!--  The comments switcher -->
-{% if page.comments and site.comments.active %}
-  {% capture path %}comments/{{ site.comments.active }}.html{% endcapture %}
+<!-- The comments switcher -->
+{% if page.comments and site.comments.provider %}
+  {% capture path %}comments/{{ site.comments.provider }}.html{% endcapture %}
   {% include {{ path }} %}
 {% endif %}