Fix image_version workflow trigger name (#41267)

Co-authored-by: Sharra-writes <sharra-writes@github.com>

Author: rhysd

content/actions/reference/workflows-and-actions/events-that-trigger-workflows.md

@@ -234,7 +234,7 @@ on:
   gollum
 ```
 
-## image_version_ready
+## `image_version`
 
 | Webhook event payload | Activity types | `GITHUB_SHA` | `GITHUB_REF` |
 |----------------------| -------------- | ------------ | -------------|