PGLog::activate_not_complete: fix log.complete_to increment typo

info.last_complete should be the entry before log.complete_to.

This appears to have been a typo introduced in
dd71051a8f6ec611b3559f8e2aff3c001be37792.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1300 from ceph/wip-7513

PGLog::activate_not_complete typo

Reviewed-by: David Zafman <david.zafman@inktank.com>