blktrace: fix blk_rq_merge documentation

The commit f3bdc62f ("blktrace: Provide event for request merging")
added the comment for blk_rq_merge() tracepoint. Remove the duplicate
word from the tracepoint documentation.

Fixes: f3bdc62f ("blktrace: Provide event for request merging")
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>