I've never seen a way to hide the SSID for the iPhone hotspot.
For years I used to hide the SSID on my home network, but I stopped. It isn't such a great security feature. If someone is really trying to be malicious, they'll simply use one of the easy to find tools that see hidden SSIDs...I found one for Macs years and years ago. Also, because of the way devices search for SSIDs, hiding an SSID can actually increase the amount of network traffic carrying the SSID name as devices seek to reconnect. Anyone eavesdropping in the area with a few basic skills or tools would see that "hidden" SSID flying around in radio packets anyway.
(Links to more info: Why hiding your SSID makes your security worse and 3 reasons why hiding your SSID is a bad idea)
For true security, hiding the SSID is not very effective, and it is much less effective than using a strong password. Fortunately, the iPhone hotspot allows you to set a strong password. After doing that, hiding the SSID would not give you any more security.