[ARIA Notify] Explainer examples refer to undefined variable

[OwenEdwards-LevelAccess]

In https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Accessibility/AriaNotify/explainer.md,

• Example 3.1,
• Example 3.2, and
• Example 3.3

each refer to a variable currentValue which is not defined in the example.

[OwenEdwards-LevelAccess]

See also WICG/accessible-notifications#2