[docs] Fix bad argument name in PTL docs (#25736)

Fixes bad argument name in PTL docs. This is just a quick fix - we should be testing the code snippet.

doc/source/ray-core/examples/using-ray-with-pytorch-lightning.rst

@@ -57,7 +57,7 @@ Here is an example of using the ``RayPlugin`` for Distributed Data Parallel trai
 
     # Create your PyTorch Lightning model here.
     ptl_model = MNISTClassifier(...)
-    plugin = RayPlugin(num_workers=4, cpus_per_worker=1, use_gpu=True)
+    plugin = RayPlugin(num_workers=4, num_cpus_per_worker=1, use_gpu=True)
 
     # If using GPUs, set the ``gpus`` arg to a value > 0.
     # The actual number of GPUs is determined by ``num_workers``.