Fix code block format in doc

[shihanng]

Hi, thank you for providing this package. While using it, I found that the code blocks in the docs seem a bit odd. One of the formatting rule that Godoc uses is:

Pre-formatted text must be indented relative to the surrounding comment text

This PR provides the simple according to the above. The following is the resulting doc rendered in HTML:

[SlyMarbo]

Thanks for this. I must've copied the text from the README and forgotten to reformat it.