doc: Update release workflow

Author: kateinoigakukun

CONTRIBUTING.md

@@ -110,7 +110,8 @@ $ git commit -m"Bump version to 0.6.0"
 $ git tag 0.6.0
 $ git push origin 0.6.0
 $ for pkg in pkg/ruby_wasm-*; do gem push $pkg; done
-$ (cd packages/gems/js/ && gem build && gem push js-*.gem)
+$ gem build && gem push ruby_wasm-*.gem && rm ruby_wasm-*.gem
+$ (cd packages/gems/js/ && gem build && gem push js-*.gem && rm js-*.gem)
 ```
 
 ## Release Channels