update-notifier: "$package" string shouldn't be translated

[Sylvie Gallet]

Hello!

> The updated .po files for update-notifier were creating some issues
> because "$package", which is a variable name, got translated in a few
> locales when it shouldn't.

   In such cases, it would be good to add a note to the translators with 
the original string saying not to translate "$package".

   Best regards,

         - Sylvie