doc: clarify that source_dir can be an S3 URI (#1108)

Author: laurenyu

src/sagemaker/estimator.py

@@ -1250,7 +1250,7 @@ def __init__(
                     >>>         |----- test.py
 
                     You can assign entry_point='src/train.py'.
-            source_dir (str): Path (absolute or relative) to a directory with
+            source_dir (str): Path (absolute, relative, or an S3 URI) to a directory with
                 any other training source code dependencies aside from the entry
                 point file (default: None). Structure within this directory are
                 preserved when training on Amazon SageMaker. If 'git_config' is