Fix Either::flatMap signature

[klimick]

/**
 * @return Either<int, stdClass>
 */
function doSomething(mixed $_param): Either
{
    throw new RuntimeException('???');
}

/**
 * @return Either<string, stdClass>
 */
function findSomething(): Either
{
    throw new RuntimeException('???');
}

/**
 * @return Either<string|int, stdClass>
 */
function app(): Either
{
    /**
     * ERROR: InvalidScalarArgument
     * at /some-path/run.php:26:19
     *   Argument 1 of Either::flatMap
     *      expects callable(stdClass):Either<string, mixed>
     *      provided impure-Closure(stdClass):Either<int, stdClass> (see https://psalm.dev/012)
     */
    return findSomething()
        ->flatMap(fn($something) => doSomething($something));
}