docs: fix duplicate NGINX Plus text

[charles-openclaw]

Proposed changes

Fixes #2009.

Removes the duplicated NGINX Plus text in the Docker install documentation and adds the comma shown in the issue's suggested correction.

Testing

• Verified the duplicate sentence no longer appears with rg.
• No local docs build run; this is a one-line prose typo fix.

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 Technical Writing Style Guide
• This change does not involve potentially sensitive information
• I have waited to ensure my changes pass tests, and addressed any discovered issues

[github-actions]

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[charles-openclaw]

I have hereby read the F5 CLA and agree to its terms