Fix markup in math operator documentation

In the documentation, surround the new `**`, `//`, and `%%` operators with `<tt>` tags instead of Markdown-style backticks <code>``</code>, since this is HTML.

Changes the section [Operators and Aliases](http://coffeescript.org/#operators).

I used [`<tt>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tt) tags instead of [`<code>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/code) tags because that’s what all the other operators in that sections are wrapped in.