Fix nullable event warning in interface events documentation

[Copilot]

The code snippet in the "How to implement interface events" documentation was generating a CS8618 nullable reference type warning when compiled with C# 8.0+ and nullable reference types enabled.

The issue occurred because the event was declared as non-nullable:

public event EventHandler ShapeChanged;

With nullable reference types enabled, the compiler warns that this event field might be null when exiting the constructor, since events are essentially multicast delegates that default to null.

This PR fixes the warning by:

1. Making the event nullable - Changed EventHandler to EventHandler? in both the inline code snippet in the markdown documentation and the corresponding code file
2. Modernizing the event invocation - Updated from explicit null checking to the null-conditional operator (ShapeChanged?.Invoke(this, e))
3. Fixing character encoding - Corrected an ellipsis character encoding issue

The changes are minimal and maintain full backward compatibility while eliminating the nullable reference type warning that would appear when developers copy and use this code in projects with nullable reference types enabled.

Fixes #29880.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/events/how-to-implement-interface-events.md	docs/csharp/programming-guide/events/how-to-implement-interface-events

[BillWagner]

This is ready for final review.