[DOC] Minor improve
SF#3610708: Fix Failing to Load DEM files on Big Endian Systems.

Endian awareness is needed in unzip_hgt_file() for processing DEM zip file headers, as the values are in little-endian byte order.
Otherwise loading on a big-endian system such as a PowerPC Mac will fail with the program complaining 'wrong format' for the DEM file.

See http://en.wikipedia.org/wiki/Zip_(file_format)

Signed-off-by: Rob Norris <rw_norris@hotmail.com>