Update README.md with new model card content

README.md

@@ -1,7 +1,7 @@
 ---
 library_name: keras-hub
 ---
-### Model Overview
+## Model Overview
 BART encoder-decoder network.
 
 This class implements a Transformer-based encoder-decoder model as
@@ -35,7 +35,7 @@ __Arguments__
     sequence length. This determines the variable shape for positional
     embeddings.
 
-### Example Usage
+## Example Usage
 ```python
 import keras
 import keras_hub