change wtheo submodule path

.gitmodules

@@ -9,4 +9,4 @@
 	url = https://git.flavigny.de/christian/theo-lecture
 [submodule "ws2020/wtheo/uebungen"]
 	path = ws2020/wtheo/uebungen
-	url = gitea:christian/wtheo-zettel
+	url = https://git.flavigny.de/christian/wtheo-zettel