Update transforming-responses.md

[Bursul]

Fixed documentation as ResponseDefinitionTransformerV2 and ResponseTransformerV2 are interfaces.

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Submitter checklist

• [x] The PR request is well described and justified, including the body and the references
• [x] The PR title represents the desired changelog entry
• [x] If the change against WireMock 2 functionality (incompatible with WireMock 3), it is submitted against the 2.x branch
• [x] The repository's code style is followed (see the contributing guide)

Details: Contributor Guide

[oleg-nenashev]

Thanks! Will review asap

[oleg-nenashev]

Thanks for catching it @Bursul !