Update cosigin verify instruction for container (updatecli#1409)

Author: olblak

README.adoc

@@ -72,8 +72,7 @@ sha256sum --ignore-missing -c checksums.txt
 **Verify Container signature**
 
 ```
-docker pull updatecli/updatecli:vx.y.z
-cosign verify updatecli/updatecli@sha256:<image_digest> 
+cosign verify --certificate-identity-regexp "https://github.com/updatecli/updatecli" --certificate-oidc-issuer "https://token.actions.githubusercontent.com" ghcr.io/updatecli/updatecli:v0.54.0
 ```
 == Documentation