examples/simple: explicitly set yaml.Loader

examples/simple.nix

@@ -31,7 +31,7 @@ in {
       - some
       - random
       - variables
-    """))
+    """, Loader=yaml.Loader))
   '';
   sed = pkgs.writeDash "sed-example" ''
     echo hello | ${pkgs.writeSed "hello-world" "s/hello/& world/"}