docs: Corrections to Inheritance and polymorphism page

[developerjamiu]

Summary

Fact-checked the Inheritance and polymorphism page. Three corrections:

• Generated code example: Showed ChildClass extends ParentClass redeclaring the parent's String name field. The analyzer explicitly forbids field redeclaration, and the actual generated code only declares new fields and forwards inherited ones via super. Removed the spurious line.
• ### Extending a Class → ### Extending a class: sentence case per STYLE_GUIDE.md.
• ### Sealed Classes → ### Sealed classes: same.

Test plan

• Render the page locally and confirm the generated code block under "Sealed classes" shows only int age on the child.

[Swiftaxe]

LGTM!