Fix code examples on "Custom drawing in 2D" page

[dalexeev]

#6455 made 2 errors in the docs:

• Missing semicolons in C# examples;
• Inconsistency in class declarations due to lack of conflicts with Update C# code samples in Getting Started and Tutorials #6581.

[raulsntos]

LGTM

[YuriSizov]

Thanks!