message: remove argument compression_type="GZIP" of mnist_io.MNISTDataset() in README.md
diff: @@ -47,8 +47,7 @@ import tensorflow_io.mnist as mnist_io
  # Read MNIST into tf.data.Dataset
  d_train = mnist_io.MNISTDataset(
  'train-images-idx3-ubyte.gz',
  - 'train-labels-idx1-ubyte.gz',
  - compression_type="GZIP")
  + 'train-labels-idx1-ubyte.gz')
  # By default image data is uint8 so conver to float32.
  d_train = d_train.map(lambda x, y: (tf.image.convert_image_dtype(x, tf.float32), y)).batch(1)
  @@ -68,7 +67,7 @@ model.fit(d_train, epochs=5, steps_per_epoch=10000)
  Note that in the above example, [MNIST](http://yann.lecun.com/exdb/mnist/) database
  files are assumed to have been downloaded and saved to the local directory.
  -Compression files could be processed automatically with `compression_type="GZIP"`.
  +Compression files(gzip) could be detected and uncompressed automatically.
  ### R Package
  