minor #15296 [Serializer] match example with description above (iamvar)

This PR was submitted for the 5.x branch but it was merged into the 4.4 branch instead.

Discussion
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[Serializer] match example with description above

Discriminator description mentions abstract class, but example contains interface

Commits
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778594c match example with description above

Author: OskarStark

components/serializer.rst

@@ -1491,7 +1491,7 @@ and ``BitBucketCodeRepository`` classes:
          *    "bitbucket"="App\BitBucketCodeRepository"
          * })
          */
-        interface CodeRepository
+        abstract class CodeRepository
         {
             // ...
         }