[Console] Fix Console component $app to $this and use of getHelper() method

| Q                  | A          |
| ---------------- |:---------:|
| Doc fix?        | Yes |
| New docs?   | No   |
| Applies to     | 2.5   |
| Fixed tickets | No   |

On Console component, I've fixed the following:

### Use of getHelper() method

```php
$this->getHelperSet()->get('dialog');
```

to:

```php
$this->getHelper('dialog');
```

### Also fixed unusued $app variable to $this

This is related to original PR https://github.com/symfony/symfony-docs/pull/3952