Correcting the estimator variable

Updating the estimator variable to match the one used in submit()
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cc: @maxluk
GitHub Issue: https://github.com/MicrosoftDocs/azure-docs/issues/47535

Author: RohitMungi-MSFT

articles/machine-learning/how-to-train-tensorflow.md

@@ -255,7 +255,7 @@ distributed_training = TensorflowConfiguration()
 distributed_training.worker_count = 2
 
 # Tensorflow constructor
-estimator= TensorFlow(source_directory=project_folder,
+tf_est= TensorFlow(source_directory=project_folder,
                       compute_target=compute_target,
                       script_params=script_params,
                       entry_script='script.py',