message: Fix a small typo of `"` => `'`
  as otherwise the example code in README.md will not run correctly
diff: @@ -24,7 +24,7 @@ import tensorflow_io as tfio
  # Read MNIST into Dataset
  d_train = tfio.IODataset.from_mnist(
  - 'http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz",
  + 'http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz',
  'http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz').batch(1)
  # By default image data is uint8 so conver to float32.
  