qa/standalone/scrub/osd-scrub-repair.sh: fix grep pattern to match decode exception

qa/standalone/scrub/osd-scrub-repair.sh: fix grep pattern to match decode exception

We fail because the error message in the log looks like:

2020-05-27T21:02:48.447+0000 7fbfc4e60700 -1 log_channel(cluster) log [ERR] : scrub 3.0 3:5c7b2c47:::ROBJ16:head : can't decode 'snapset' attr void SnapSet::decode(ceph::buffer::v15_2_0::list::const_iterator&) no longer understand old encoding version 3 < 97: Malformed input

Fixes: https://tracker.ceph.com/issues/45660
Signed-off-by: Neha Ojha <nojha@redhat.com>