iio: potentiometer: Fix typo in Kconfig

Fix s/potentiomenter/potentiometer/.

Suggested-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>