static: T4283: fix missing f'ormat string
This fixes the error message:
Can not use both blackhole and reject for prefix "{prefix}"!
Added in commit bb78f3a9ad28 ("static: T4283: support "reject" routes - emit an
ICMP unreachable when matched")
(cherry picked from commit 490ee3ec5ba7ea28002890841eab8e46f775a129)