docs(form): replace obsolete tt element

Removes the [**obsolete** HTML Teletype Text Element `<tt>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/tt)
and replaces it with [`<code>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/code).
This adds more semanticity and is part of the [HTML5 specification](http://www.w3.org/TR/html5/text-level-semantics.html#the-code-element).

Closes #11570