doc: fix confusion markdown in util.markdown

Caught this one in reading the util doc, checked the doc of nodej and iojs:

• nodejs: https://nodejs.org/api/util.html#util_util_format_format
• iojs: https://iojs.org/api/util.html#util_util_format_format

But the github markdown worked at https://github.com/iojs/io.js/blob/v1.x/doc/api/util.markdown#utilformatformat-

Well, now I fixed the confusion bug on on this line, but btw, I think we should keep using the markdown parser same to github markdown, so that we needn't another review on website. I'm interested in this proposal if someone could give me the guiding link on how the website are built and working, Thank you :)