update optimizer_step example in docs (#10420)

Author: rohitgr7

pytorch_lightning/core/lightning.py

@@ -1564,15 +1564,15 @@ def optimizer_step(
                 using_native_amp,
                 using_lbfgs,
             ):
-                # warm up lr
+                # update params
+                optimizer.step(closure=optimizer_closure)
+
+                # manually warm up lr without a scheduler
                 if self.trainer.global_step < 500:
                     lr_scale = min(1.0, float(self.trainer.global_step + 1) / 500.0)
                     for pg in optimizer.param_groups:
                         pg["lr"] = lr_scale * self.learning_rate
 
-                # update params
-                optimizer.step(closure=optimizer_closure)
-
         """
         optimizer.step(closure=optimizer_closure)