remove duplicated directory separator
remove duplicated directory separator

The $rootPath property is already initialized with a trailing directory
separator in the class constructor.

nicolas-grekas/Twig,twigphp/Twig,nicolas-grekas/Twig,twigphp/Twig,nicolas-grekas/Twig,twigphp/Twig,nicolas-grekas/Twig,nicolas-grekas/Twig