Sync doc/stringio in sync_default_gems.rb

Author: peterzhu2118

tool/sync_default_gems.rb

@@ -272,6 +272,7 @@ def lib((upstream, branch), gemspec_in_subdir: false)
       ["ext/stringio", "ext/stringio"],
       ["test/stringio", "test/stringio"],
       ["stringio.gemspec", "ext/stringio/stringio.gemspec"],
+      ["doc/stringio", "doc/stringio"],
     ], exclude: [
       "ext/stringio/README.md",
     ]),