What does "heat the oven to broil" mean?
In general, there are two types of ovens:


Ovens with a broiler drawer under their oven.
Ovens with a second top heating element (broiler) in the main oven compartment.


Your oven is the former; mine (and apparently the author's) is the latter.

So, in this case, the instructions mean "heat the oven to the broil setting".

Since you have the first type of oven, your only option will be to put your oven in broiler mode and hope the distance between the broiler and the fish is sufficient. With the recipe calling for the fish to be on the middle rack, it sounds like it's pretty far from the heat source, so you'll probably need to keep a close eye on your fish as I'm guessing your setup will be much closer. It's even possible that it won't work at all.