Fixes avrodoc code blocks

This is the smallest possible change to fix the random `code` blocks that are appearing in our docs. 

Essentially, if a newline has 4 spaces in front of it, avrodoc thinks thats a markdown code block. 

Long term, I think we need to fix our style guide to indent avro records by 2 spaces only, and then shift the bodies of the /** */ comments over by 1 space, so that a paragraph only ever has 3 spaces in front of it. (or - someone can patch avrodoc with a better fix :)