Update custom plugin code example

[offa]

Replaces the JSR 305 @Nonnull annotations in the custom plugin code example.

Testing done

None – docs only.

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4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[uhafner]

Thanks for spotting!