scsi: documentation: Fix case of 'scsi_device' struct mention(s)

In scsi_mid_low_api.txt a the scsi_device structure is mentioned
several times, but the leading 's' is uppercase (Scsi_device)
and should be lowercase (scsi_device).  Fixed by this commit.

Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>