migrate the godoc badge to pkg.go.dev

[shogo82148]

Issue #, if available:

N/A

Description of changes:

migrate links for godoc.org to pkg.go.dev.
These badges keep working (ref. Redirecting godoc.org requests to pkg.go.dev), but users will get better experience by skipping redirections.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #366 (36b1308) into master (6c2af88) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #366   +/-   ##
=======================================
  Coverage   72.22%   72.22%           
=======================================
  Files          19       19           
  Lines         738      738           
=======================================
  Hits          533      533           
  Misses        138      138           
  Partials       67       67           

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[harrisonhjones]

LGTM