That should be set by the layer's name, either from the web map or the feature service. If you look at this popup sample, that heading is "US Counties". If you look at the web map used in the sample, you'll see that the layer has the title of "US Counties".
For a sample using a service, the heading has "ACS Marital Status Boundaries - Tract", the service name and layer name.
The empty space next to your symbol for that Untitled layer can have text that is set by the PopupTemplate's title property.