tag's prefix is "v"

tool/format-release

@@ -178,7 +178,7 @@ eom
     if /\.0(?:-\w+)?\z/ =~ ver
       # preview, rc, or first release
       entry <<= <<eom
-  tag: ruby_#{ver.tr('.-', '_')}
+  tag: v#{ver.tr('.-', '_')}
   stats:
     files_changed: #{files_changed}
     insertions: #{insertions}