docs: it_IT: maintainer-pgp-guide: Fix reference to "Nitrokey Pro 2"

This fixes the following Sphinx warning:

Documentation/translations/it_IT/process/maintainer-pgp-guide.rst:458:
  WARNING: Unknown target name: "nitrokey pro".

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>