Update TensorFlow examples following API change (#44)

Author: nkconnor

README.rst

@@ -718,7 +718,7 @@ follows:
 
   from sagemaker.tensorflow import TensorFlow
 
-  tf_estimator = TensorFlow('tf-train.py', role='SageMakerRole',
+  tf_estimator = TensorFlow(entry_point='tf-train.py', role='SageMakerRole',
                             training_steps=10000, evaluation_steps=100,
                             train_instance_count=1, train_instance_type='ml.p2.xlarge')
   tf_estimator.fit('s3://bucket/path/to/training/data')
@@ -1042,7 +1042,7 @@ The following code sample shows how to train a custom TensorFlow script 'tf-trai
 
   from sagemaker.tensorflow import TensorFlow
 
-  tf_estimator = TensorFlow('tf-train.py', role='SageMakerRole',
+  tf_estimator = TensorFlow(entry_point='tf-train.py', role='SageMakerRole',
                             training_steps=10000, evaluation_steps=100,
                             train_instance_count=1, train_instance_type='ml.p2.xlarge')
   tf_estimator.fit('s3://bucket/path/to/training/data')