Update NGINX certificate file extension

[mjang]

Proposed changes

F5 WAF for NGINX docs incorrectly refers to files with the .cert extension (they should be files with the .crt extension)

Closes #1391

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. ↩

[JTorreG]

@ohad-perets this MUST be approved by a WAF SME before merge

[github-actions]

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