Update ObservableObject.xml

[PerfidusPistor]

wrong description for PropertyChanging

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Learn Build status updates of commit 57dd1ae:

✅ Validation status: passed

File	Status	Preview URL	Details
dotnet/xml/Microsoft.Toolkit.Mvvm.ComponentModel/ObservableObject.xml	✅Succeeded	View

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[prmerger-automator]

PRMerger Results

Issue	Description
Unapproved file extension(s)	This PR contains file types that are not supported in the repo. The only supported file types are jpg, gif, png, json, svg and md.

[learn-build-service-prod]

Learn Build status updates of commit 57dd1ae:

✅ Validation status: passed

File	Status	Preview URL	Details
dotnet/xml/Microsoft.Toolkit.Mvvm.ComponentModel/ObservableObject.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[prmerger-automator]

PRMerger Results

Issue	Description
Unapproved file extension(s)	This PR contains file types that are not supported in the repo. The only supported file types are jpg, gif, png, json, svg and md.