MAINTAINERS: ftm-quaddec: Fix typo in a filepath

Fix typo (s/quadddec/quaddec/) in the path to the documentation.

Cc: linux-iio@vger.kernel.org
Fixes: 517b2d04 ("MAINTAINERS: add counter/ftm-quaddec driver entry")
Signed-off-by: Denis Efremov <efremov@linux.com>
Acked-by: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>