Fix broken fragment link to #operator-token

[Copilot]

Link checker failed on #operator-token fragment in /influxdb/v2/admin/tokens/. The newly created _index.md used h4 headings which lychee couldn't reliably detect.

Changed token type headings from h4 to h3:

• #### Operator token → ### Operator token
• #### All Access token → ### All Access token
• #### Read/Write token → ### Read/Write token

Anchor IDs unchanged; all existing links remain valid.

• Signed the InfluxData CLA
  (if necessary)
• Rebased/mergeable

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix broken link</issue_title>
<issue_description>Reported by link-checker in #6585:

Error: Broken link: file:///home/runner/work/docs-v2/docs-v2/public/influxdb/v2/admin/tokens#operator-token - Cannot find fragment: Fragment not found in document. Check if fragment exists or page structure

</issue_description>

<agent_instructions>Check the files in the mentioned PR, find and fix the broken link</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix broken link #6608

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