Fix bug in checklist template
Fix bug in checklist template

Template links to non-existent action: `site_stats`
This causes a test failure:
```
ObserverControllerTest#test_checklist_for_site:
ActionView::Template::Error: No route matches {:action=>"site_stats", :controller=>"observer"}
    app/views/observer/checklist.html.erb:51:in `_app_views_observer_checklist_html_erb__1875846783326497009_81845440'
    app/controllers/application_controller.rb:205:in `catch_errors'
    test/functional_test_case.rb:24:in `get'
    test/controllers/observer_controller_test.rb:3750:in `test_checklist_for_site'

```
Solution: correct the action name

(I don't know how this test ever passed. Perhaps some magic in dynamic links in test mode.)

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