Update openapi.md

[walva]

Use openapiContext (object, documentation) instead of openapi (boolean, enabled or not)

Based on what I see here : https://github.com/api-platform/core/blob/3.2/src/Metadata/Post.php

https://api-platform.com/docs/3.2/core/openapi/ @ "Changing Operations in the OpenAPI Documentation"

[walva]

Based on what I see here : https://github.com/api-platform/core/blob/3.2/src/Metadata/Post.php