main/ndisc6: fix regex to no longer depend on sed

One of the regular expressions in the merge-hook is not valid, and
busybox sed throws an error on it. Fixing it is trivial and it allows us
to drop the sed dependency.
7 jobs for !13738 with ndisc6-to-main in 1 minute and 25 seconds
detached
Status Job ID Name Coverage
  Lint
passed #235682
docker-alpine x86_64
lint

00:00:24

 
  Build
passed #235687
docker-alpine ci-build aarch64
build-aarch64

00:00:37

passed #235688
docker-alpine ci-build armv7
build-armv7

00:00:44

passed #235686
docker-alpine ci-build ppc64le
build-ppc64le

00:00:59

passed #235685
docker-alpine ci-build s390x
build-s390x

00:00:40

passed #235684
docker-alpine ci-build x86
build-x86

00:00:29

passed #235683
docker-alpine ci-build x86_64
build-x86_64

00:00:33