fix PropertyChangedMessage<T> link

[WebFreak001]

fixes a copy paste issue with the link being the same as the one below

[opbld31]

Docs Build status updates of commit 63f8a2d:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/mvvm/Introduction.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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