docs: Corrections to Endpoint inheritance page

[developerjamiu]

Summary

Fact-checked the Endpoint inheritance page. Two corrections:

• Code example bug: super.hello(...) → await super.greet(...) (wrong method name + missing await on a Future<String>).
• @doNotGenerate parent: corrected wording to match actual generator output (no parent client class generated; child extends EndpointRef directly with parent methods inlined).

Verified by running serverpod generate on a real project.

Test plan

• Render the page locally and confirm both code blocks read correctly.