Fix a wrongly generated doc for to many relations

[Rocksheep]

When a relation what specified as a to many relationship it would generate a wrong documentation for the detach endpoint.
Before my fix it expected an object instead of an array.

Before this fix:

{
    "data": {
        "type": "tags",
        "id": "<string>"
    }
}

After this fix:

{
    "data": [
        {
            "type": "tags",
            "id": "<string>"
        },
        {
            "type": "tags",
            "id": "<string>"
        }
    ]
}