Fix for #2485

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | #2485

Fixes the documented example of how to test forms with custom types,
as there was no better suggestions than the one put forward by
@chrisburgess7 I implemented that in my test case & it worked so
I've updated the documentation to match.