message: fixes estimator model_fn signature in python docs
diff: @@ -99,7 +99,7 @@ class EstimatorSpec(
  ignored in eval and infer modes. Example:
  ```python
  - def my_model_fn(mode, features, labels):
  + def my_model_fn(features, labels, mode):
  predictions = ...
  loss = ...
  train_op = ...
  @@ -114,7 +114,7 @@ class EstimatorSpec(
  given mode. Example:
  ```python
  - def my_model_fn(mode, features, labels):
  + def my_model_fn(features, labels, mode):
  if (mode == tf.estimator.ModeKeys.TRAIN or
  mode == tf.estimator.ModeKeys.EVAL):
  loss = ...
  