Update ObservableObject.cs

[PerfidusPistor]

Closes #1438

PR Checklist

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

Other information

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

[echoix]

I think before being ready, this PR would have to have a clean title and maybe a bit more context, as the issue referred to doesn't exist in the repo.