Update ko/mkdocs.yml for a missing link.

Other translations are in `자습서 - 사용자안내서`,
but only the translation of `tutorial/encoder.md` in
'Tuturoal - User Guide'.

Because ko/nkdocs.yml doesn't include `tutorial/encoder.md`.

docs/ko/mkdocs.yml

@@ -66,6 +66,7 @@ nav:
   - tutorial/response-status-code.md
   - tutorial/request-files.md
   - tutorial/request-forms-and-files.md
+  - tutorial/encoder.md
 markdown_extensions:
 - toc:
     permalink: true