Placeholders: Convert `type` correctly

[phillippbertram]

"list_item_done_ago": "done {when}",
    "@list_item_done_ago": {
        "description": "Small caps date when item was marked as done.",
        "placeholders": {
            "when": {
                "type": "String", // <---- this
                "example": "a minute ago"
            }
        }
    }