Fix json fields not working

[plewin]

Hello,
In the doc there is the following example

"tables":{
    "subscriptions":{
        "additional_data":{
            "action":"json_update",
            "fields":[
                {
                    "path":"$.user.first_name",
                    "type":"firstname"
                },
                {
                    "path":"$.user.last_name",
                    "type":"lastname"
                },
                {
                    "path":"$.user.phone",
                    "type":"telephone"
                }
            ]
        }
    }
}

I could not get this to work with master's code.

"tables":{
    "subscriptions":{
        "additional_data":{
            "action":"json_update",
            "field":
                {
                    "path":"$.user.first_name",
                    "type":"firstname"
                }
            ]
        }
    }
}

While could work.

I am not sure which is correct, the documentation or the code.
I have modified in my code to make the documentation example works.