Small typo - wrong date format

[hacfi]

9.3. Lifecycle Callbacks

    public function doStuffOnPrePersist()
    {
        $this->createdAt = date('Y-m-d H:m:s');
    }

https://github.com/doctrine/orm-documentation/blame/master/en/reference/events.rst#L243

H:m:s should be H:i:s

[hacfi]

See PR #141