Update getting-started.rst

The former require declaration couldn't resolve to a package.

```txt
  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,
```

Author: felds

cookbook/creating_a_cms/getting-started.rst

@@ -29,7 +29,7 @@ all of the required packages now.
         ...
         require: {
             ...
-            "symfony-cmf/routing-auto-bundle": "1.0.0@alpha",
+            "symfony-cmf/routing-auto-bundle": "1.0.0-alpha10 as 1.0.0",
             "symfony-cmf/menu-bundle": "1.0",
             "sonata-project/doctrine-phpcr-admin-bundle": "dev-master",
             "doctrine/data-fixtures": "1.0.0",