Ouch, poor Zelda! I'm not sure if the cut is deep from the picture, but it is long. The best thing to do is having her seen by a vet, as it may need stitches to help it heal properly. In addition, wounds from dog bites can frequently become infected and she could use antibiotics to prevent infection. I would gently clean the area with warm water and antiseptic soap, withhold food for tonight (she will need to be sedated if she requires stitches), and bring her into the vet in the morning.