Remove lead slash

Addresses FriendsOfPHP/PHP-CS-Fixer#705
@keradus I do not want to change it. I just wanted a clarification about its meaning. It is named as get*Namespace*UseIndexes and I get T_USE outside of namespaces. Indeed, you pass true to its parameter, you get groups of T_USE per namespace, but no indication whatsoever that a particular group actually belongs to the global namespace.

I am fixing this issue differently.