Update error_reporting.md

`User.find_by(params[:id])` will raise `ArgumentError` as it expects hash / keyword argument.

I believe the point of this example was to show that `user` will be assigned to `User.anonymous` anytime User with id from `params[:id]` does not exist in the database.

Author: lcmen

guides/source/error_reporting.md

@@ -88,7 +88,7 @@ If no error is raised in the block, `Rails.error.handle` will return the result
 
 ```ruby
 user = Rails.error.handle(fallback: -> { User.anonymous }) do
-  User.find_by(params[:id])
+  User.find(params[:id])
 end
 ```