message: Fix typos in Linear Model Tutorial samples
  1. test_file_name is undefined (should be test_file.name)
  2. train_file_name is undefined (should be train_file.name)
diff: @@ -383,7 +383,7 @@ API:
  ```python
  # set num_epochs to None to get infinite stream of data.
  m.train(
  - input_fn=input_fn(train_file_name, num_epochs=None, shuffle=True),
  + input_fn=input_fn(train_file.name, num_epochs=None, shuffle=True),
  steps=train_steps)
  ```
  @@ -392,7 +392,7 @@ the labels of the holdout data:
  ```python
  results = m.evaluate(
  - input_fn=input_fn(test_file_name, num_epochs=1, shuffle=False),
  + input_fn=input_fn(test_file.name, num_epochs=1, shuffle=False),
  steps=None)
  print("model directory = %s" % model_dir)
  for key in sorted(results):
  