block: fix spelling and grammar for in writeback_cache_control.rst

Suggested-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20240619154623.450048-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>