fix: php 8.3 release page example code

[kamilpiech97]

In that section about php 8.3 release, I found mistake in example of code and I fixed them.

Before PR:

 function getBytesFromString(string $string, int $length) {
    $stringLength = strlen($string);

    $result = '';
    for ($i = 0; $i < $length; $i++) {
        // random_int is not seedable for testing, but secure.
        $result .= $string[random_int(0, $stringLength - 1)]);
    }

    return $result;
}

After PR:

function getBytesFromString(string $string, int $length) {
    $stringLength = strlen($string);

    $result = '';
    for ($i = 0; $i < $length; $i++) {
        // random_int is not seedable for testing, but secure.
        $result .= $string[random_int(0, $stringLength - 1)];
    }

    return $result;
}

[saundefined]

Good catch, thanks!