Proposed keyserver changed to functional one

[Vitexus]

Fixes #990 - the not so much experienced users are confused by cryptic error message

Requirements

This is only minor change. No extra requrements

Description of the Change

The current example uses pool.sks-keyservers.net that is no longer operating. The new one is working well:

repo@apt:~/bin$ gpg --no-default-keyring --keyring trustedkeys.gpg --keyserver pool.sks-keyservers.net --recv-keys CE1A3DD5E3C94F49
gpg: keyserver receive failed: No name
repo@apt:~/bin$ 
repo@apt:~/bin$ gpg --no-default-keyring --keyring trustedkeys.gpg --keyserver keyserver.ubuntu.com --recv-keys CE1A3DD5E3C94F49
gpg: key CE1A3DD5E3C94F49: public key "MariaDB Enterprise Signing Key <signing-key@mariadb.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1

[lbolla]

Thanks!