docs(outputs.dynatrace): Use TOML comment in api_token example

[jstirnaman]

Summary

The api_token example used a C-style // comment, which is 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; after, the
config loads (Loaded inputs: cpu).

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 api_token example used a C-style // comment, which is invalid in TOML. Switching to # is correct: TOML uses #, and an inline # comment after a value parses fine. README-only prose block, so no sample.conf edit or make docs needed.