OCS OpenAPI tutorial: Mention that the parent class of the API has to end with OCSController

[dartcafe]

Before You Submit

• I have checked for existing issues and pull requests related to my suggestion.
• I have posted on the Nextcloud Community Help Forum if my suggestion is a troubleshooting scenario or I'm unsure about the solution.

Type of Suggestion

Mention that the name of the parent class of the ApiController has to end with OCSController.

Relevant Documentation Links

https://docs.nextcloud.com/server/latest/developer_manual/client_apis/OCS/ocs-openapi.html

Additional Notes

I got mad about this, because I had a baseApiV2Controller class (now renamed baseApiV2Controller) until I stumbled over the comment inside the OpenAPIExtractor. I did not understand the error message, because I was extending OCSController, but inside the parent class.