Rename remaining Dutch "nationaalgeoregister" PDOK WMTS URLs

[justb4]

This PR relates to issues #226 and #392 (PR #401).

A code search showed 3 remaining URLs to be adapted in docs and test code. This PR modifies these:

• From: "https://geodata.nationaalgeoregister.nl/tiles/service/wmts/brtachtergrondkaart/EPSG:28992/{z}/{x}/{y}.png"
• To: "https://service.pdok.nl/brt/achtergrondkaart/wmts/v2_0/standaard/EPSG:28992/{z}/{x}/{y}.png"

See also PDOK.