Fix for create_function deprecated in PHP 7.2

On line 652 of rest-manager/includes/class-rest-manager-settings.php, it currently says:

$callback = create_function('', 'echo "'.str_replace('"', '\"', $section['desc']).'";');

create_function is deprecated in PHP 7.2, so this line should be changed to:

$callback = function() {
	echo str_replace('"', '\"', $section['desc']);
};