Small error in README.md

There is:
```php
$cache = \Fabfuel\Prophiler\Decorator\Phalcon\Cache\BackendDecorator($cacheBackend, $profiler);
```

There should be:
```php
$cache = new \Fabfuel\Prophiler\Decorator\Phalcon\Cache\BackendDecorator($cacheBackend, $profiler);
```