Update header with cosign instructions

Author: maraino

.goreleaser.yml

@@ -200,8 +200,7 @@ release:
 
     ```
     cosign verify-blob \
-      --certificate step-ca_darwin_{{ .Version }}_amd64.tar.gz.pem \
-      --signature step-ca_darwin_{{ .Version }}_amd64.tar.gz.sig \
+      --bundle step-ca_darwin_{{ .Version }}_amd64.tar.gz.sigstore.json \
       --certificate-identity-regexp "https://github\.com/smallstep/workflows/.*" \
       --certificate-oidc-issuer https://token.actions.githubusercontent.com \
       step-ca_darwin_{{ .Version }}_amd64.tar.gz