systemd-systemctl: fix errors in instance name expansion

If the instance name indicated by %i begins with a number, the meaning of the
replacement string "\\1{}".format(instance) is ambiguous.

To indicate group number 1 regardless of the instance name, use "\g<1>".

Signed-off-by: Yuta Hayama <hayama@lineo.co.jp>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>