Fix a small typo (ht: masterkain)

actionpack/lib/abstract_controller/layouts.rb

@@ -347,7 +347,7 @@ def _default_layout(require_layout = false)
       begin
         layout_name = _layout if action_has_layout?
       rescue NameError => e
-        raise e.class, "Coult not render layout: #{e.message}"
+        raise e.class, "Could not render layout: #{e.message}"
       end
 
       if require_layout && action_has_layout? && !layout_name