lib/recursiveUpdateUntil: fix code to match documentation

$ nix repl lib
Welcome to Nix version 2.0.2. Type :? for help.

Loading 'lib'...
Added 350 variables.

-- this is the exact example from the function's documentation:
nix-repl> recursiveUpdateUntil (path: l: r: path == ["foo"]) {
                   # first attribute set
                   foo.bar = 1;
                   foo.baz = 2;
                   bar = 3;
                 } {
                   #second attribute set
                   foo.bar = 1;
                   foo.quz = 2;
                   baz = 4;
                 }
{ bar = 3; baz = 4; foo = { bar = 1; baz = 2; quz = 2; }; }

-- although the documentation says:
{
    foo.bar = 1; # 'foo.*' from the second set
    foo.quz = 2; #
    bar = 3;     # 'bar' from the first set
    baz = 4;     # 'baz' from the second set
}