Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
fix README for admission webhook test image
This README is copied from somewhere else and it's out of date.
**Release note**:
```release-note
NONE
```
ecc5eb67d9