BindNReturn documentation error

The [documentation](https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/computation-expressions) for BindNReturn gives this as its signature:

```
(M<'T1> * M<'T2> * ... * M<'TN> * ('T1 * 'T2 ... * 'TN -> M<'U>)) -> M<'U>
```

In fact, I believe the signature should be:

```
(M<'T1> * M<'T2> * ... * M<'TN> * ('T1 * 'T2 ... * 'TN -> 'U)) -> M<'U>
```

That is, the supplied function returns a pure `'U`. The examples given in the [sample ApplicativeBuilder](https://github.com/fsharp/fslang-design/blob/main/FSharp-5.0/FS-1063-support-letbang-andbang-for-applicative-functors.md) also agree that the supplied function returns pure values.