Fix endblock (#517)

ref
https://app.circleci.com/pipelines/github/readthedocs/readthedocs.org/11313/workflows/e5fa3898-bc7c-4668-9271-8ebf386c5b42/jobs/29013

Author: stsewd

readthedocsext/theme/templates/gold/partials/projects_list.html

@@ -7,7 +7,7 @@
 
 {% block list_placeholder_class %}
   ui short placeholder segment
-{% endblock list_placeholder_icon_class %}
+{% endblock list_placeholder_class %}
 {% block list_placeholder_icon %}
 {% endblock list_placeholder_icon %}
 {% block list_placeholder_header %}