Provide a 'Changelog' link on rubygems.org/gems/racc

By providing a 'changelog_uri' in the metadata of the gemspec a
'Changelog' link will be shown on https://rubygems.org/gems/racc
which makes it quick and easy for someone to check on the changes
introduced with a new version.

Details of this functionality can be found on https://guides.rubygems.org/specification-reference/

racc.gemspec

@@ -44,6 +44,7 @@ DESC
  s.required_ruby_version = ">= 2.5"
  s.rdoc_options = ["--main", "README.rdoc"]
  s.extra_rdoc_files = ["README.ja.rdoc", "README.rdoc"]
+ s.metadata["changelog_uri"] = "https:/ruby/racc/releases"
 
  if RUBY_PLATFORM =~ /java/
  s.files << 'lib/java/racc/cparse-jruby.jar'