Failure to dispose CancellationToken in "How To: Listen for Multiple Cancellation Requests"

It seems as though `WorkerWithTimer.internalToken` is never disposed in this example code. My understanding is that `CancellationTokens` needs to be disposed? Generally speaking this is something that could be spelled out more explicitly in `CancellationToken` docs.

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 073f1486-9fa6-a17c-d649-d4ae5a36b8d8
* Version Independent ID: 55cc3625-d0a5-3422-0d27-945823a0c64b
* Content: [How to: Listen for Multiple Cancellation Requests](https://docs.microsoft.com/en-us/dotnet/standard/threading/how-to-listen-for-multiple-cancellation-requests)
* Content Source: [docs/standard/threading/how-to-listen-for-multiple-cancellation-requests.md](https://github.com/dotnet/docs/blob/main/docs/standard/threading/how-to-listen-for-multiple-cancellation-requests.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**