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>

Documentation/block/stat.rst

@@ -18,7 +18,7 @@ A.
    each, it would be impossible to guarantee that a set of readings
    represent a single point in time.
 
-The stat file consists of a single line of text containing 11 decimal
+The stat file consists of a single line of text containing 17 decimal
 values separated by whitespace.  The fields are summarized in the
 following table, and described in more detail below.