How can I convert a 220v lighting circuit to 110v?
Note: This assumes you are in the US or an area that follows US electrical standards. If you are someplace else then this may be totally incorrect.

You need to connect hot, neutral, ground. Currently you have a second hot instead of neutral.

The catch is that this requires moving a wire at the panel. Assuming you have "normal" wiring, you should have white for neutral, green or bare wire for ground and some other color, usually black, for hot.

If the cable from the panel to the fixture is black/white/green-or-bare then you need to move white from a hot connection to the neutral bar. WARNING: If you have never worked inside a breaker panel before then this is probably NOT a good idea to do yourself.

If the cable from the panel is black/red/green-or-bare (i.e., designed for a 240V circuit with no neutral) then the cable should be switched to one that includes a white wire.

If the cable from the panel is black/red/white/green-or-bare (like for a stove or dryer or other circuit) then you should already have neutral in the cable (the white wire) and either the wires were connected wrong at the panel or at the fixture - should have white to neutral on both ends.

My personal recommendation (I am not an electrician) is to hire an electrician to do this. But probably a different electrician because while it should be "you messed up, you fix it", I would worry that if they messed up on such a basic thing that they really can't be trusted to fix it properly. In particular, the comment someone else made about "connect to one 110 VAC and ground" is a bit scary: Doing that would make the light work properly, but would absolutely violate code and would not be safe. If your "electrician" decided to do a quick fix like that then I'd be really worried...