[Documentation] Wrong PHP version

[JonGarbayo]

Hi!

Thanks for that simple but great extension :)
Just a note: the latest 2.0.0 version can only work with PHP 7.0 and above, due to the use of the null coalescing operator (??) in the getPluralizedString function.

You should bump the minimum PHP version in the composer.json file, or use a PHP 5 compliant operator :)

Have a nice day!