IntroToRx Documentation Partitioning.Buffer Code May Be Wrong

(I *really* appreciate all of your work on this. I'd do a "PR" but I don't use git and don't really feel comfortable trying to do a PR here as my first one ever.)

#### Bug / Documentation Typo

Intro To Rx documentation 3rd edition Updated for Rx.NET v6.1

08_Partitioning.md

In the buffering example, the code is

```
IObservable> shipStatusChanges =
perShipObservables.SelectMany(shipMessages => shipMessages
.OfType()
.DistinctUntilChanged(m => m.NavigationStatus)
.Buffer(2, 1));

IDisposable sub = shipStatusChanges.Subscribe(m => Console.WriteLine(
$"Ship {((IAisMessage)m[0]).Mmsi} changed status from" +
$" {m[1].NavigationStatus} to {m[1].NavigationStatus}" +
$" at {DateTimeOffset.UtcNow}"));
```

I think it should be

```
IObservable> shipStatusChanges =
perShipObservables.SelectMany(shipMessages => shipMessages
.OfType()
.DistinctUntilChanged(m => m.NavigationStatus)
.Buffer(2, 1));

IDisposable sub = shipStatusChanges.Subscribe(m => Console.WriteLine(
$"Ship {((IAisMessage)m[0]).Mmsi} changed status from" +
$" {m[0].NavigationStatus} to {m[1].NavigationStatus}" +
$" at {DateTimeOffset.UtcNow}"));
```

That is, the line

```
$" {m[1].NavigationStatus} to {m[1].NavigationStatus}" +
```

should have the first array subscript changed from 1 to 0. As I understand it, the Buffer will pair consecutive samples so m[0] should be the old state and m[1] should be the new state so it should be

```
$" {m[0].NavigationStatus} to {m[1].NavigationStatus}" +
```

Open 08_Partitioning.md and locate the buffering example in the Intro To Rx documentation. Verify that the message describes a transition from m[0] to m[1], then correct the duplicated array subscript; the example should show the previous and new navigation statuses in that order.

Written by the indexing model from the issue text.