Update environment variables from NMS to NGINX_AGENT

[spencerugbo]

Proposed changes

Update the environment prefix seen in the documentation from NMS to NGINX_AGENT

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• I have read the CONTRIBUTING document
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• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[ADubhlaoich]

LGTM!