Fix docs typo: train_batch => val_batch

Author: EspenHa

pytorch_lightning/core/lightning.py

@@ -408,7 +408,7 @@ def validation_step(self, *args, **kwargs) -> Dict[str, Tensor]:
             # the pseudocode for these calls
             val_outs = []
             for val_batch in val_data:
-                out = validation_step(train_batch)
+                out = validation_step(val_batch)
                 val_outs.append(out)
                 validation_epoch_end(val_outs)