dgit-maint-debrebase(7): Drop bad advice about upstream/ tag case

dgit-maint-debrebase(7): Drop bad advice about upstream/ tag case

The first argument to the new-upstream subcommand is a version number,
not a tag name, so this instruction could never have been correct.

The user should not need usually to pass both the upstream version
number and the upstream/ tag name, either, because git-debrebase
should find it for them.

Reported-by: David Bremner <bremner@debian.org>
Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>