Update docs to reflect rojo#574

[Wingysam]

Trying to define Attributes as the docs previously stated like

{
  "properties": {
    "Attributes": {
      "key": "value"
    }
  }
}

results in a "data did not match any variant of untagged enum UnresolvedValue" error.

In rojo-rbx/rojo#574, the process for defining attributes was changed. This PR updates the docs to reflect that.