docs: use adapter factory shorthand (adapter: vercelAdapter) (#409)

Update docs and examples to pass the adapter factory directly
(adapter: vercelAdapter) instead of calling it (adapter: vercelAdapter()),
reflecting the AdapterOrFactory support in flag(). Applied consistently to
all zero-arg factory adapters (vercel, edge-config, custom adapter examples).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>