docs: block: fix stat.rst document error

Update the description of the device stat files to include the proper
number of fields.

Signed-off-by: Gao Mingfei <g199209@gmail.com>
Link: https://lore.kernel.org/r/20210601085145.3273-1-g199209@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>