Update Show magnifier sample readme to correctly reflect API functionality

[duffh]

Description

Updated Show magnifier sample to demonstrate correct usage of mapView.MapViewInteractionOptions.IsMagnifierEnable and the default property to be true.

Type of change

• Other enhancement

Platforms tested on

• WPF .NET 8
• WPF Framework
• WinUI
• MAUI WinUI
• MAUI Android
• MAUI iOS
• MAUI MacCatalyst

Checklist

• Self-review of changes
• All changes work as expected on all affected platforms
• There are no warnings related to changes
• Code is commented and follows .NET conventions and standards
• Codemaid and XAML styler extensions have been run on every changed file
• No unrelated changes have been made to any other code or project files
• Screenshots are correct size and display in description tab (800 x 600 on Windows, MAUI mobile platforms use the MAUI Windows screenshot)

[williambohrmann3]

good catch, approving pending readme style check resolution (they need end of line characters)