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.

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[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)	⬆️

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[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[aledbf]

@onesolpark thanks!