documentation: add fourth input type to type list for inputs in doc string

Author: ajberdy

src/sagemaker/estimator.py

@@ -650,8 +650,9 @@ def fit(self, inputs=None, wait=True, logs="All", job_name=None, experiment_conf
         model using the Amazon SageMaker hosting services.
 
         Args:
-            inputs (str or dict or sagemaker.inputs.TrainingInput): Information
-                about the training data. This can be one of four types:
+            inputs (str or dict or sagemaker.inputs.TrainingInput or
+                sagemaker.inputs.FileSystemInput): Information about the training data.
+                This can be one of four types:
 
                 * (str) the S3 location where training data is saved, or a file:// path in
                     local mode.