docs: replace outdated WMS server in r.out.vtk example

[rohannallamadge]

Replaced the dead WMS server with https://gibs.earthdata.nasa.gov/wms/epsg3857/best/wms.cgi.

Updated the layer used in the example to BlueMarble_ShadedRelief_Bathymetry.

Adjusted the region settings using g.region to ensure proper visualization.

Updated commands for fetching RGB data (r.in.wms) and exporting it to VTK format (r.out.vtk).

[pesekon2]

Please update also the title of the example. Currently, it says Spearfish example with RGB data. However, that's not what the example is about and the Spearfish location is in a different projection making your example not working due to reprojection. If you wish, you could use even your own projection in the example if you specify it so the user knows how to reproduce the example.