Update THIRD-PARTY-NOTICES.md

THIRD-PARTY-NOTICES.md

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-- `diagrams/xamarin-forms-webassembly-renderer.*` is based off https://forums.xamarin.com/discussion/170673/xamarin-architecture-diagram-updated/ by Xamarin.
+- `diagrams/xamarin-forms-webassembly-renderer.*` is based on https://forums.xamarin.com/discussion/170673/xamarin-architecture-diagram-updated/ by Xamarin.