Add required override to the Streamable example in reference docs [DATACMNS-1837]

[spring-projects-issues]

Mark Paluch opened DATACMNS-1837 and commented

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Referenced from: pull request #475

Backported to: 2.4.2 (2020.0.2), 2.3.6 (Neumann SR6), 2.2.12 (Moore SR12)