Use .pub extension for public keys in sigstoreSigned examples
Use .pub extension for public keys in sigstoreSigned examples

Hopefully users understand public/private keys enough not
to distribute the private ones, but let's minimize opportunities
for making that mistake.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image