Adjust protoc-gen-openapiv2/main.go flags docs as suggested during #1832 review

[zoido]

Have you read the Contributing Guidelines?

yes

Brief description of what is fixed or changed

There were bunch stylistic and grammatical suggestions for the parameters docs in the #1832 by @johanbrandhorst.
This PR applies the suggestions also in the protoc-gen-openapiv2/main.go flags description as that;s the place where the text was copied from.

[codecov-io]

Codecov Report

Merging #1833 (d36459f) into master (370d869) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1833   +/-   ##
=======================================
  Coverage   57.39%   57.39%           
=======================================
  Files          34       34           
  Lines        3652     3652           
=======================================
  Hits         2096     2096           
  Misses       1290     1290           
  Partials      266      266           

Impacted Files	Coverage Δ
protoc-gen-openapiv2/main.go	24.54% <ø> (ø)

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[johanbrandhorst]

Thanks for your contribution!