docs: rename stripe-webhook-secret example to webhook-secret

Removes the lingering external-company reference in the named-value
secrets example. Elizabeth's earlier commit renamed the token to
`WEBHOOK_SECRET` but the named value itself was still called
`stripe-webhook-secret`; this aligns the name with the generic token.

Follow-up to review feedback on PR #166.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Peter Hauge

docs/guides/environment-overrides.md

@@ -142,7 +142,7 @@ namedValues:
       displayName: payment-api-key
       secret: true
       value: "{#[PAYMENT_API_KEY]#}"  # Replaced at pipeline runtime
-  - name: stripe-webhook-secret
+  - name: webhook-secret
     properties:
       secret: true
       value: "{#[WEBHOOK_SECRET]#}"