minor #15302 [Serializer] update example for attributes (iamvar)

This PR was submitted for the 5.x branch but it was merged into the 5.2 branch instead.

Discussion
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[Serializer] update example for attributes

update attributes example as well
see #15296

Commits
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19b8d23 update example for attributes

Author: javiereguiluz

components/serializer.rst

@@ -1608,7 +1608,7 @@ and ``BitBucketCodeRepository`` classes:
             'github' => GitHubCodeRepository::class,
             'bitbucket' => BitBucketCodeRepository::class,
         ])]
-        interface CodeRepository
+        abstract class CodeRepository
         {
             // ...
         }