Fix ownership syntax in example: root.root → root:root

[BlakeRhodes]

This PR corrects a small syntax issue in a code example where file ownership was written as:

sudo mv ~/.yubico/challenge-123456 /var/yubico/aaronh-serial
sudo chown root.root /var/yubico/aaronh-serial
sudo chmod 600 /var/yubico/aaronh-serial

Changed . to :

sudo mv ~/.yubico/challenge-123456 /var/yubico/aaronh-serial
sudo chown root:root /var/yubico/aaronh-serial
sudo chmod 600 /var/yubico/aaronh-serial

[jacobgkau]

Thanks for the typo correction!

[thomas-zimmerman]

Again, thank you for the typo fix!