GPX docs. example missing -nlt point in -sql example

[tbnorth]

At the end of

​http://www.gdal.org/ogr/drv_gpx.html

an example uses -sql to remap field names for GPX, but this doesn't work without -nlt point (or whatever).

See #4190 - adding -nlt point made it work, just as rouault said.

[Even Rouault]

r22923 /trunk/gdal/ogr/ogrsf_frmts/gpx/drv_gpx.html: GPX: add a FAQ section to explain more clearly the meaning and solution to an error message (#4191)

Note: the example is correct. For shapefiles, the SQL engine used is the OGR SQL one and it can find the geometry type of the original layer used by the SQL request. In the postgres case, the SQL engine used is the native postgres one.