Documentation: block/diskstats: update function names

__make_request() and end_that_request_last() do no longer exist. Replace
them with the current call-site.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviwed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220222012751.1933194-1-naohiro.aota@wdc.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>