Typo fix "api server " -> "API server"

[AdamDang]

In this doc ,"API server" is written in capitals, but line 64 is not.

[k8s-reviewable]

This change is 

[codecov-io]

Codecov Report

Merging #2165 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2165      +/-   ##
==========================================
+ Coverage   36.44%   36.58%   +0.14%     
==========================================
  Files          69       69              
  Lines        4862     4862              
==========================================
+ Hits         1772     1779       +7     
+ Misses       2820     2814       -6     
+ Partials      270      269       -1

Impacted Files	Coverage Δ
cmd/nginx/main.go	26.2% <0%> (+4.82%)	⬆️

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[aledbf]

/approve

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