typo in admissionregistration in pre-1.8

Hi,

There is a typo in admissionregistration. I know it is now registered via the code but somebody (like me) may find it useful. I fixed it here https://github.com/tivvit/example-webhook-admission-controller/tree/fix_admissionregistration but I cannot make the pull request to tag, so I suggest moving it to branch and then I will do the pull request.

Thanks