Fix configure_optimizers example (#20420)

Co-authored-by: Alan Chu <alanchu@Alans-Air.lan>

examples/fabric/build_your_own_trainer/run.py

@@ -41,7 +41,8 @@ class MNISTModule(L.LightningModule):
 
     def configure_optimizers(self):
         optim = torch.optim.Adam(self.parameters(), lr=1e-4)
-        return optim, {
+        return {
+            "optimizer": optim,
             "scheduler": torch.optim.lr_scheduler.ReduceLROnPlateau(optim, mode="max", verbose=True),
             "monitor": "val_accuracy",
             "interval": "epoch",