Fixed English for user already existing.

Core/User/InMemoryUserProvider.php

@@ -56,7 +56,7 @@ public function __construct(array $users = array())
     public function createUser(UserInterface $user)
     {
         if (isset($this->users[strtolower($user->getUsername())])) {
-            throw new \LogicException('Another user with the same username already exist.');
+            throw new \LogicException('Another user with the same username already exists.');
         }
 
         $this->users[strtolower($user->getUsername())] = $user;