Fix repository URL in README.md

[pocke]

This pull request updates a link to this repository in README.

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I also checked other default gems that ware published since Ruby 2.7, and confirmed the all of the other repositories have the correct URL in README. I picked up the repositories from NEWS-2.7.0.

• The following default gems were published on rubygems.org

  • benchmark
  • cgi
  • delegate
  • getoptlong
  • net-pop
  • net-smtp
  • open3
  • pstore
  • readline
  • readline-ext
  • singleton

  https://github.com/ruby/ruby/blob/7f3efee1024ba46189aa4a65d08bb87fdcf1bb24/doc/NEWS-2.7.0#label-Stdlib+compatibility+issues+-28excluding+feature+bug+fixes-29