docs(inputs.opcua_listener): Use TOML comments in group example

[jstirnaman]

Summary

The node_ids group example used C-style // comments, which are invalid in
TOML. Use # so the snippet parses if copied as-is.

This fix is in a freestanding example block in the README prose, not the
generated @sample.conf block (which is separate in this README and
unaffected), so no sample.conf change or make docs run is needed.

Found while adding code-block linting to the InfluxData docs site, which
generates the published plugin docs from these READMEs.

Verified in Telegraf 1.39.1: before, line 4: invalid TOML syntax on the //;
after, the block passes TOML parsing.

Note: the same example also uses an outdated inline-array form for node_ids,
tracked separately in #19189 and out of scope here.

Checklist

• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

Related to #19201 (tracking issue for this cleanup; item resolved in this PR)

[skartikey]

@jstirnaman The node_ids group example used C-style // comments, which are invalid in TOML. Switching the three lines to # is correct: TOML uses #, and inline # comments after array elements parse fine. README-only prose block, so no sample.conf edit or make docs needed.

[srebhan]

@jstirnaman can you please restore the PR description template, especially the AI boxes?!

[jstirnaman]

@jstirnaman can you please restore the PR description template, especially the AI boxes?!

@srebhan Done for all 5 PRs. Apologies.