maintainers: add materus to maintainer-list

maintainers/maintainer-list.nix

···
     githubId = 7878181;
     name = "Mateo Diaz";
   };
+  materus = {
+    email = "[e-mail]";
+    github = "materusPL";
+    githubId = 28183516;
+    name = "Mateusz Słodkowicz";
+  };
   math-42 = {
     email = "[e-mail]";
     github = "Math-42";