documentation: Correct comment in SKLearn Estimator about default Python version (aws#1385)

Author: sandeepmistry

src/sagemaker/sklearn/estimator.py

@@ -80,7 +80,7 @@ def __init__(
                 and values, but ``str()`` will be called to convert them before
                 training.
             py_version (str): Python version you want to use for executing your
-                model training code (default: 'py2'). One of 'py2' or 'py3'.
+                model training code (default: 'py3'). One of 'py2' or 'py3'.
             image_name (str): If specified, the estimator will use this image
                 for training and hosting, instead of selecting the appropriate
                 SageMaker official image based on framework_version and