Doc: Docbook/iio: Fix typo in iio.tmpl

This patch fix a spelling typo in iio.tmpl.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>