Replace keyserver.net by an active keyserver URL

[ThomasHedden]

File: [nitrokey3/linux/openpgp-keygen-backup.rst] https://docs.nitrokey.com/nitrokey3/linux/openpgp-keygen-backup.html
In the URL:
https://docs.nitrokey.com/nitrokey3/linux/openpgp-keygen-backup.html#key-import
In the section:
Uploading the Public Key
It says you can upload a keyID with this command:
$ gpg --keyserver search.keyserver.net --send-key keyID
However, the site keyserver.net is no longer active. Please replace this URL with a working keyserver.
Thomas Hedden