Example code using an invalid pattern?

### Type of issue

Typo

### Description

The last 2 examples on the Null Values page use an invalid pattern match pattern `?:`. It doesn't compile for me, nor is it listed on [Pattern Matching](https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/pattern-matching).
Those examples are:

```fsharp
match x with
| ?: string | null -> ...
```
and
```fsharp
match x with
| ?: string
| null -> ...
```

### Page URL

https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/values/null-values

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/fsharp/language-reference/values/null-values.md

### Document Version Independent Id

514581e9-9a8d-3771-928a-5d079c809a42

### Platform Id

626f2dae-4409-1f6f-5da3-ba1bc2b2d59f

### Article author

@BillWagner

### Metadata

* ID: 5ae21b87-1505-41b6-8fc9-1cfa59c90ed5
* PlatformId: 626f2dae-4409-1f6f-5da3-ba1bc2b2d59f
* Service: **dotnet-fsharp**

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