Update ObservableObject.cs

[PerfidusPistor]

Closes #1438

PR Checklist

• Based off latest main branch of toolkit
• Tested code with current supported SDKs
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Now the method code ObservableObject.SetProperty follows what it's written in documentation