Documentation: GID code example rename tileAt to get_tile

[eishiya]

Updated the example C++ code to use "get_tile" as the hypothetical method to get a tile based on its ID instead of "tileAt", for two reasons:

1. The rest of the code uses snake_case, but this one used camelCase for some reason.
2. "tileAt" sounds like it implies that the ID is a location/index. This is misleading, since in Tiled tilesets, the ids are not necessarily positions or indices - tiles may be stored out of order, and the IDs may not even be sequential. I think "get tile" avoids such implications.

As far as I can tell, this shouldn't affect any localisations, as no translations touch the sample code.

[bjorn]

Thanks!

Next time I'm adjusting my existing C++ code, I should probably just ask some AI to convert the naming style. It will do a better job than me. :P