Fix usage metadata in hello plugin examples

[fallintoplace]

Summary

Fix the example hello-world plugins so their Clipanion usage metadata is attached to the expected static field.

Root cause

Both example commands used static usageusage = Command.Usage(...) instead of static usage = Command.Usage(...), so Clipanion never reads the intended help metadata.

Fix

• rename usageusage to usage in scripts/plugin-hello-world.js
• rename usageusage to usage in scripts/plugin-hello-universe.js

Verification

• confirmed the typo was present in both files and nowhere else in these examples
• confirmed the resulting diff is only the two field renames