blktrace: fix documentation for blk_fill_rw()

blktrace: fix documentation for blk_fill_rw()

Add missing ":" after rwbs function parameter documentation that fixes
following warning :-

./kernel/trace/blktrace.c:1877: warning: Function parameter or member 'rwbs' not described in 'blk_fill_rwbs'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: d406061cceea ("blktrace: add blk_fill_rwbs documentation comment")
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>