Fix indentation in tasks.h

Change-Id: I5cba82df5d3e04a13022d6fab3849e677dbe7a2c
Reviewed-on: http://review.couchbase.org/38340
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>