message: fix the function name in the gide of custom estimator
diff: @@ -149,7 +149,7 @@ model. This configuration step is similar to how we configured the @{tf.estimato
  ```python
  classifier = tf.estimator.Estimator(
  - model_fn=my_model,
  + model_fn=my_model_fn,
  params={
  'feature_columns': my_feature_columns,
  # Two hidden layers of 10 nodes each.
  @@ -474,7 +474,7 @@ Instantiate the custom Estimator through the Estimator base class as follows:
  ```python
  # Build 2 hidden layer DNN with 10, 10 units respectively.
  classifier = tf.estimator.Estimator(
  - model_fn=my_model,
  + model_fn=my_model_fn,
  params={
  'feature_columns': my_feature_columns,
  # Two hidden layers of 10 nodes each.
  