maintainers: update nullcube

Changed GitHub username

Author: nullcubee

maintainers/maintainer-list.nix

@@ -16961,7 +16961,7 @@
   nullcube = {
     email = "[email protected]";
     name = "NullCube";
-    github = "nullcub3";
+    github = "nullcubee";
     githubId = 51034487;
     matrix = "@nullcube:matrix.org";
   };