[13.x] Fix StartSession docblock

[oliverquynh]

Let's look at StartSession::cache method, its return docblock is invalid because \Illuminate\Cache\Store doesn't exist.

Actually, in runtime, call_user_func($this->cacheFactoryResolver) must return an instance of CacheManager, so the expected returned value must be an implementation of \Illuminate\Contracts\Cache\Repository interface.

/**
 * Resolve the given cache driver.
 *
 * @param  string  $driver
 * @return \Illuminate\Contracts\Cache\Repository
 */
protected function cache($driver)
{
    // call_user_func($this->cacheFactoryResolver) returns an instance of CacheManager
    // CacheManager::driver() returns an implementation of \Illuminate\Contracts\Cache\Repository interface
    return call_user_func($this->cacheFactoryResolver)->driver($driver);
}