docs: fix missed Steward→Org Admin in API key expiration note

The key rotation note in the Maintainer steps block incorrectly said
"the Steward will initiate rotation" — key rotation is an Org Admin
action, consistent with the rest of the PSGallery key section.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: HeyItsGilbert

docs/maintainer-onboarding.md

@@ -72,7 +72,7 @@ The Org Admin then:
 ### Maintainer steps
 
 - [ ] Confirm with the Org Admin that `PSGALLERY_API_KEY` is set in the repo's Actions secrets (**Settings** → **Secrets and variables** → **Actions**)
-- [ ] Note the key expiration month — the Steward will initiate rotation, but you may be asked to trigger it
+- [ ] Note the key expiration month — the Org Admin will initiate rotation, but you may be asked to trigger it
 
 ### Key rotation