Fix the install command

[Mailaender]

as it won't install without this parameter.

[popey]

I don't think we should do this to any classic snaps.

If you try to install eclipse without specifying --classic then you receive this information:

alan@ziggy:~$ snap install eclipse
error: This revision of snap "eclipse" was published using classic confinement
       and thus may perform arbitrary system changes outside of the security
       sandbox that snaps are usually confined to, which may put your system at
       risk.

       If you understand and want to proceed, repeat the command including
       --classic.

If we add --classic to the instructions, we're removing this informational message. We should not assume non-expert new users to this system will know what the implications of this parameter are.