Update synopsis for “gnt-cluster repair-disk-sizes”

Mention that instances can be passed on the CLI when “--help” is used.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Andrea Spadaccini <spadaccio@google.com>
(cherry picked from commit eb5ac108)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>