Fix comment stating isSubmitted is optional

[smnandre]

The isSubmitted() must be called before

src/Symfony/Component/Form/Form.php

    public function isValid(): bool
    {
        if (!$this->submitted) {
            throw new LogicException('Cannot check if an unsubmitted form is valid. Call Form::isSubmitted() and ensure that it\'s true before calling Form::isValid().');
        }

        if ($this->isDisabled()) {
            return true;
        }

        return 0 === \count($this->getErrors(true));
    }

[javiereguiluz]

Another good catch! Thanks Simon 🙌