zebra: Add missing new line for help string

zebra: Add missing new line for help string

```
  -A, --asic-offload        FRR is interacting with an asic underneath the linux kernel
      --v6-with-v4-nexthops Underlying dataplane supports v6 routes with v4 nexthops  -s, --nl-bufsize          Set netlink receive buffer size
```

Fixes: 1f5611c06d1c243b42279748788f0627793ead9c ("zebra: Allow zebra cli to accept v6 routes with v4 nexthops")
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>
(cherry picked from commit 25ae643996d338b8230fb15a9064843fe85de224)