docs: Corrections to Working with endpoints page

[developerjamiu]

Summary

Fact-checked the Working with endpoints page against the Serverpod 3.5.0-beta.4 source. Three corrections:

• Removed broken $localhost interpolation in the client init example.
• Reworded the Android emulator paragraph that assumed localhost was a Dart variable.
• Aligned default request size with the configuration page: 524288 bytes (512 KiB), doubled to 1 MiB.

Test plan

• Render the page locally and confirm the code block, emulator paragraph, and warning callout read correctly.

[Swiftaxe]

LGTM!