minor #25765 Add missing @ in phpdoc return statement (grongor)

This PR was submitted for the master branch but it was merged into the 3.3 branch instead (closes #25765).

Discussion
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Add missing @ in phpdoc return statement

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
-------

efb8fa1 Add missing @ in phpdoc return statement

src/Symfony/Component/Cache/Adapter/AdapterInterface.php

@@ -31,7 +31,7 @@ interface AdapterInterface extends CacheItemPoolInterface
     /**
      * {@inheritdoc}
      *
-     * return \Traversable|CacheItem[]
+     * @return \Traversable|CacheItem[]
      */
     public function getItems(array $keys = array());
 }