docs: spell out HMAC on first use per vale lint rule

Author: alicenstar

docs/vendor/event-notifications-create.mdx

@@ -53,7 +53,7 @@ To create an event notification subscription:
          - Must respond with a 2xx status code (200-299) for successful delivery
          - Response time must be less than five seconds
 
-      1. (Recommended) Enter a signing secret for HMAC signature verification. For more information, see [Verify webhook signatures](event-notifications-webhooks#verify-webhook-signatures) in _Configure and test webhooks_.
+      1. (Recommended) Enter a signing secret for Hash-based Message Authentication Code (HMAC) signature verification. For more information, see [Verify webhook signatures](event-notifications-webhooks#verify-webhook-signatures) in _Configure and test webhooks_.
 
       1. (Optional) Expand **Advanced configuration** to add custom HTTP headers for authenticating with your endpoint (for example, `Authorization: Bearer token`). For more information, see [About adding custom HTTP headers](event-notifications-webhooks#custom-headers) in _Configure and test webhooks_.