Editorial change to 3.4.2 Example

[johnfoliot]

The current code example shows the following:

However, the current list of allowable token values is simplets, medium, or low (there is no "critical").

Additionally, there is currently no such attribute known as "aria-function" - this should be removed from the example. (source: https://www.w3.org/TR/wai-aria-1.1/)

[becka11y]

removed aria-function from example code; changed simplest to critical in the value descriptions; did some rework of the wording.

Removed: Typically 3 links or buttons would be considered "simplest" - I didn't understand what it meant and it referred to simplest rather than critical.

[clapierre]

Closed via #86