Add example of iconOffset in API

This PR:
* renames MonFIchier.txt to data.txt in order to be consistent with the project using english as main language.
* adds example of iconOffset in this example txt data file.

Actually the added offset are the default ones anyway:
https://github.com/openlayers/openlayers/blob/master/lib/OpenLayers/Format/Text.js#L59-L60
but at least it makes the example more explict on what to do when using icons with another size.