Fix error loading gemspec

[sato11]

Installing dependencies has been failing since the commit 41ccfee introduced an undefined local variable in the gemspec:

[!] There was an error parsing `Gemfile`:
[!] There was an error while loading `rack-cors.gemspec`: undefined local variable or method `s' for main:Object. Bundler cannot continue.

 #  from /home/runner/work/rack-cors/rack-cors/rack-cors.gemspec:22
 #  -------------------------------------------
 #
 >    s.metadata = {
 #      'changelog_uri' => 'https://github.com/cyu/rack-cors/blob/master/CHANGELOG.md'
 #  -------------------------------------------
. Bundler cannot continue.

 #  from /home/runner/work/rack-cors/rack-cors/Gemfile:6
 #  -------------------------------------------
 #  # Specify your gem's dependencies in rack-cors.gemspec
 >  gemspec
 #
 #  -------------------------------------------