Mg/kg is milligrams of the active ingredient in the medication, given for every kilogram of the rabbits body weight. So with liquid medication like oral suspensions or injectable meds, there is the concentration of active ingredient, in this case azithromycin, measured in milligrams, in a milliliter of the liquid. So if you have an azithromycin suspension that is, let's say, 40mg/ml, then that would mean there is 40mg of the antibiotic in a ml of the suspension liquid. The vet determines the amount, or mg, of the medication that a rabbit needs per kg of body weight, and takes the amount of the concentration of the med in the bottle being used(mg/ml) and calculates the needed dose.
So to know if the dose you were given is correct or not, you would need to know the concentration(mg/ml) of the medication that you were given.
Something to also be aware of with azithromycin if your vet didn't discuss it, is that it can cause a loss of appetite in some instances. So it would be good to also be prepared to syringe feed your rabbit in case this happens, if this isn't already something you are doing.