i2c: slave: fix the example how to instantiate from userspace

I copied the wrong shell code into the documentation. Sorry to all who
tried to get sense out of this current example :/ Slight rewording while
we are here.

Reported-by: Tim Bakker <bakkert@mymail.vcu.edu>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org