Fix version (#173)

Found during heroku/languages-github-actions#87

buildpacks/ruby/CHANGELOG.md

@@ -6,7 +6,7 @@
 - Commons: Remove `gem_list`, `rake_status`, `rake_task_detect` modules (https://github.com/heroku/buildpacks-ruby/pull/155)
 - Commons: `EnvCommand` removed, replaced with `fun_run` (https://github.com/heroku/buildpacks-ruby/pull/139)
 
-## [2.0.0] 2023/31/01
+## [2.0.0] 2023/01/31
 
 - Initial version of Ruby buildpack in Rust (https://github.com/heroku/buildpacks-ruby/pull/93)
 - Version 2.0.0 for the first release is not a typo. There was an issue in pack where a builder with the same name and version number would reuse artifacts left on image from [prior runs which caused issues](https://github.com/buildpacks/pack/issues/1322#issuecomment-1038241038). There were prior releases of `heroku/ruby` CNB from different sources that triggered this problem. To ensure no one would encounter that issue we developed and released using a version we know has not been used before. Version 2.0 was the first major version without a prior release of `heroku/ruby` CNB from any source.