You have to nest the fields inside set in {"options":{"fields":[...]}}
{
"fields": [
{
"type": "text",
"label": "Text"
},
{
"type": "html",
"label": "Html Code"
},
{
"type": "set",
"label": "Multiple",
"options": {
"fields": [
{
"name": "name",
"type": "text"
},
{
"name": "about",
"type": "html"
}
]
}
}
]
}