Distributed tracing with links has wrong signature

Hey there, I was checking on how to add links to distributed tracing and found out this [1] part which sets `parentContext` as null but, there is no signature in `StartActivity` where `parentContext` is optional, did I miss something?
Trying the following will return an error:
```csharp
_activitySource.StartActivity(ActivityKind.Consumer, parentContext: null);
```
```
Argument type 'null' is not assignable to parameter type 'System.Diagnostics.ActivityContext'
```

[1] https://docs.microsoft.com/en-us/dotnet/core/diagnostics/distributed-tracing-instrumentation-walkthroughs#optional-links

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* ID: 3f30f3b0-90cb-d2eb-ee08-4319ffcb7686
* Version Independent ID: dc4689df-b03a-e744-dbed-f23d8defcb6e
* Content: [Add distributed tracing instrumentation - .NET](https://docs.microsoft.com/en-us/dotnet/core/diagnostics/distributed-tracing-instrumentation-walkthroughs)
* Content Source: [docs/core/diagnostics/distributed-tracing-instrumentation-walkthroughs.md](https://github.com/dotnet/docs/blob/main/docs/core/diagnostics/distributed-tracing-instrumentation-walkthroughs.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @tommcdon
* Microsoft Alias: **tommcdon**