fix typo in pix2pix_model.py

udpate -> update

Author: eltociear

Diff for: models/pix2pix_model.py

@@ -124,4 +124,4 @@ def optimize_parameters(self):
         self.set_requires_grad(self.netD, False)  # D requires no gradients when optimizing G
         self.optimizer_G.zero_grad()        # set G's gradients to zero
         self.backward_G()                   # calculate graidents for G
-        self.optimizer_G.step()             # udpate G's weights
+        self.optimizer_G.step()             # update G's weights