backups: cosmetic: Rename .inc file to .html

Tests performed:

- Go to backups page.

- Take a backup and see it listed in list of repositories.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/backups/templates/backups.html

@@ -58,7 +58,7 @@
   <h3>{% trans 'Existing Backups' %}</h3>
 
   {% for repository in repositories %}
-    {% include "backups_repository.inc" with uuid=repository.uuid %}
+    {% include "backups_repository.html" with uuid=repository.uuid %}
   {% endfor %}
 
 {% endblock %}