Update getting-started.rst

The former require declaration couldn't resolve to a package.

```
  Problem 1
    - The requested package symfony-cmf/routing-auto-bundle could not be found in any version, there may be a typo in the package name.
```

Fixed by declaring the exact version.

PS: I also think it's worth mentioning that the `composer.json` file must have these directives to work properly:

```json
{
    "minimum-stability": "dev",
    "prefer-stable": true,
}
```