Update Orca 5min (#6524)

docs/readthedocs/source/doc/Orca/Overview/orca.md

@@ -50,6 +50,7 @@ Finally, use [sklearn-style Estimator APIs in Orca](distributed-training-inferen
 ```python
 from bigdl.orca.learn.tf2.estimator import Estimator
 
+# Define the NCF model in standard TensorFlow API
 def model_creator(config):
     from tensorflow import keras
 
@@ -79,6 +80,8 @@ val_steps = int(test_df.count() / batch_size)
 
 est = Estimator.from_keras(model_creator=model_creator, backend="spark",
                            config={"embed_dim": 8, "num_users": num_users, "num_items": num_items})
+
+# Distributed training
 est.fit(data=train_df,
         batch_size=batch_size,
         epochs=4,
@@ -87,6 +90,8 @@ est.fit(data=train_df,
         steps_per_epoch=train_steps,
         validation_data=test_df,
         validation_steps=val_steps)
+
+# Distributed inference
 prediction_df = est.predict(test_df,
                             batch_size=batch_size,
                             feature_cols=['user', 'item'],