Update docs to avoid html-special-characters filtering

[moreal]

I resolved issue #783

In my thought, this problem was occured from using syntax not standard
(The docs build program might think < and > can be used for xss attack.)

So I changed the code block format from ":::" => "<lang> using code (gist)

Please check it's successfully changed by me, link, netmq-forked readthedocs

[somdoron]

thanks!