iio: Fix typos in the struct iio_event_spec documentation comments

This patch fixes a few minor typos in the documentation comments for the
scan_type member of the iio_event_spec structure. The sign member name
was improperly capitalized as "Sign" in the comments. The storagebits
member name was improperly listed as "storage_bits" in the comments. The
endianness member entry in the comments was moved after the repeat
member entry in order to maintain consistency with the actual struct
iio_event_spec layout.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>