Fix Open API doc for DateTime requirements
Fix Open API doc for DateTime requirements

When using the following requierement:

```
@Rest\QueryParam(name="updatedFrom", strict=true, nullable=true, requirements=@Constraints\DateTime("Y-m-d\TH:i:s"))
```

According to the OpenAPI 3.0 documentation https://swagger.io/specification/, we should have "format": "date-time" rather than "format": "DateTime" in the generated doc
nelmio/NelmioApiDocBundle,nelmio/NelmioApiDocBundle,nelmio/NelmioApiDocBundle