Update gpu warning (Lightning-AI#6181)

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Kaushik Bokka <kaushikbokka@gmail.com>

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -544,7 +544,10 @@ def set_distributed_mode(self, distributed_backend: Optional[str] = None):
         rank_zero_info(f'TPU available: {_TPU_AVAILABLE}, using: {num_cores} TPU cores')
 
         if torch.cuda.is_available() and self._device_type != DeviceType.GPU:
-            rank_zero_warn("GPU available but not used. Set the --gpus flag when calling the script.")
+            rank_zero_warn(
+                "GPU available but not used. Set the gpus flag in your trainer"
+                " `Trainer(gpus=1)` or script `--gpus=1`."
+            )
 
     def _set_horovod_backend(self):
         self.check_horovod()