Fixed typo in flags.go

[onesolpark]

Noticed a small typo while changing different configs for ssl-passthrough.
Flags.go, ssl-passtrough-proxy-port -> ssl-passthrough-proxy-port.

What this PR does / why we need it:
Fixed typo in flags.go (ssl-passthrough-proxy-port)

Special notes for your reviewer:
Not really a bug or anything but I guess I had to let you know just in case.

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: helpdesk@rt.linuxfoundation.org

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[onesolpark]

/check-cla

[codecov-io]

Codecov Report

Merging #2932 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #2932   +/-   ##
=======================================
  Coverage   47.56%   47.56%           
=======================================
  Files          77       77           
  Lines        5531     5531           
=======================================
  Hits         2631     2631           
  Misses       2560     2560           
  Partials      340      340

Impacted Files	Coverage Δ
cmd/nginx/flags.go	83.33% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e7b8c2e...63aef5d. Read the comment docs.

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, onesolpark

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aledbf]

@onesolpark thanks!