Proofread cmd package and update flags description

[antoineco]

What this PR does / why we need it:

Originally part of #2368.

[antoineco]

/assign @aledbf
/assign @ElvinEfendi

[codecov-io]

Codecov Report

Merging #2623 into master will increase coverage by 0.2%.
The diff coverage is 76%.

@@            Coverage Diff            @@
##           master    #2623     +/-   ##
=========================================
+ Coverage   40.68%   40.88%   +0.2%     
=========================================
  Files          75       75             
  Lines        5108     5129     +21     
=========================================
+ Hits         2078     2097     +19     
- Misses       2751     2753      +2     
  Partials      279      279

Impacted Files	Coverage Δ
cmd/nginx/main.go	23.13% <13.04%> (-0.36%)	⬇️
cmd/nginx/flags.go	83.33% <94.8%> (+2.21%)	⬆️

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[aledbf]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[aledbf]

@antoineco thanks!