update example code on TFF front page: dummy_batch was deprecated from from_keras_model()

docs/_index.yaml

@@ -69,10 +69,6 @@ landing_page:
         # Pick a subset of client devices to participate in training.
         train_data = [client_data(n) for n in range(3)]
 
-        # Grab a single batch of data so that TFF knows what data looks like.
-        sample_batch = tf.nest.map_structure(
-            lambda x: x.numpy(), iter(train_data[0]).next())
-
         # Wrap a Keras model for use with TFF.
         def model_fn():
           model = tf.keras.models.Sequential([
@@ -81,7 +77,7 @@ landing_page:
           ])
           return tff.learning.from_keras_model(
               model,
-              dummy_batch=sample_batch,
+              input_spec=train_data[0].element_spec,
               loss=tf.keras.losses.SparseCategoricalCrossentropy(),
               metrics=[tf.keras.metrics.SparseCategoricalAccuracy()])