blktrace: fix blk_rq_issue documentation

The commit 881245dc ("Add DocBook documentation for the block tracepoints.")
added the comment for blk_rq_issue() tracepoint. Remove the duplicate
word from the tracepoint documentation.

Fixes: 881245dc

 ("Add DocBook documentation for the block tracepoints.")
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>