Fix render: file call

rails/rails#35688 deprecated calling `render file` and letting it lookup
the file from anywhere. This was deprecated in 6.0 and removed in 6.1,
which is why travis was failing for new PRs.

Author: eileencodes

test/dummy/app/controllers/template_assertions_controller.rb

@@ -20,7 +20,7 @@ def render_file_absolute_path
   end
 
   def render_file_relative_path
-    render file: 'test/dummy/README.rdoc'
+    render file: '/test/dummy/README.rdoc'
   end
 
   def render_with_layout