Array and string offset access syntax with curly braces is deprecated

[gerardo1sanchez]

https://github.com/gneustaetter/XLSXReader/blob/10cad4e800adc1f7793c4bbfb2dd1e730f0735b7/XLSXReader.php#L252

Small sugestion to be compatible with PHP 8 (remove curly braces)

$index += (ord($col[$i]) - 64) * pow(26, $colLen-$i-1);