docs: capitalize K in word koa

[gaomd]

It's Koa.

I think we should have a consistent branding, especially in official docs, hence I have capitalized all K's in word koa (in docs), package names and variables in docs remain untouched of course.

[codecov]

Codecov Report

Merging #1126 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1126   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           5        5           
  Lines         372      372           
=======================================
  Hits          371      371           
  Misses          1        1

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