Fix mailinglist url for mainz

[dreua]

Fixup for #707, thanks @moschlar and @bhousel for the initial addition and review ❤️

Currently wrong mailing list url: https://lists.openstreetmap.org/listinfo/mainz (org)
Correct mailing list url: https://lists.openstreetmap.de/listinfo/mainz (de)
Preview: https://openstreetmap.community/?map=49.79900,8.31769&zoom=8.87

I used Stuttgart as a template:

osm-community-index/resources/europe/germany/de-stuttgart-mailinglist.json

Lines 7 to 11 in a85e277

	"strings": {
	"name": "Stuttgart and Baden-Wuerttemberg Mailing List",
	"description": "This is the mailing list for the OSM community of Stuttgart and whole Baden-Wuerttemberg",
	"url": "https://lists.openstreetmap.de/listinfo/stuttgart"
	},

[bhousel]

Thanks @dreua , looks good 👍