Updated "Models defined by data" (#3433)

* Updated "Models defined by data"

* Fixed WARNING: Inline substitution_reference start-string without end-string.

Co-authored-by: David Waterworth <david.waterworth@cim.io>

docs/source/introduction_guide.rst

@@ -289,15 +289,15 @@ When your models need to know about the data, it's best to process the data befo
     dm.setup()
 
     model = LitModel(out_features=dm.num_classes, img_width=dm.img_width, img_height=dm.img_height)
-    trainer.fit(model)
+    trainer.fit(model, dm)
 
 
 1. use `prepare_data` to download and process the dataset.
 2. use `setup` to do splits, and build your model internals
 
-|
+An alternative to using a DataModule is to defer initialization of the models modules to the `setup` method of your LightningModule as follows:
 
-.. testcode::
+.. code-block:: python
 
     class LitMNIST(LightningModule):