[Serializer] Correct typing mistake in DocBlock

| Q             | A
| ------------- | ---
| Branch?       | 2.1 to 4.0
| Bug fix?      | yes (comment only)
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | na
| License       | MIT
| Doc PR        |

DocBlock comment referred to `NormalizableInterface` but code was using `DenormalizableInterface`