in formdemo example need update composer.json and lock file for PHP 7.3

[russeljo]

Need update dependecies to
"zendframework/zend-session": "^2.8",

now it's
"zendframework/zend-session": "^2.7.1",

an cause "composer install" problem.
issue of zend-session zendframework/zend-session#74 fixed in 2.8 version