updated readme for TF 2.2
updated readme for TF 2.2

GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst,GoogleCloudPlatform/training-data-analyst