Use appropriate Content-Type header in examples

[pdabelf5]

Proposed changes

• Some examples for publishing JSON content to the NIM security policy APIs was missing the appropriate Content-Type header, resulting in the command failing.
• Other cases, the syntax for adding the header was incorrect.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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