undo unnecessary docstring change

Author: laurenyu

src/sagemaker/mxnet/estimator.py

@@ -53,7 +53,7 @@ def __init__(self, entry_point, source_dir=None, hyperparameters=None, py_versio
             source_dir (str): Path (absolute or relative) to a directory with any other training
                 source code dependencies aside from tne entry point file (default: None). Structure within this
                 directory are preserved when training on Amazon SageMaker.
-            hyperparameters (dict): Hyperparameters that will be used for training (default: {}).
+            hyperparameters (dict): Hyperparameters that will be used for training (default: None).
                 The hyperparameters are made accessible as a dict[str, str] to the training code on SageMaker.
                 For convenience, this accepts other types for keys and values, but ``str()`` will be called
                 to convert them before training.