docs(readme): add Korean to the documentation language bar (#8836)

The Korean docs went live with QwenLM/qwen-code-docs#212, so
/ko/users/overview now resolves. Until now the language bar could not
list Korean because that path 404ed.

Uses the native name 한국어, matching the other entries, and appends it
last so the existing order is untouched.

README.md

@@ -14,7 +14,8 @@
 <a href="https://qwenlm.github.io/qwen-code-docs/fr/users/overview">français</a> |
 <a href="https://qwenlm.github.io/qwen-code-docs/ja/users/overview">日本語</a> |
 <a href="https://qwenlm.github.io/qwen-code-docs/ru/users/overview">Русский</a> |
-<a href="https://qwenlm.github.io/qwen-code-docs/pt-BR/users/overview">Português (Brasil)</a>
+<a href="https://qwenlm.github.io/qwen-code-docs/pt-BR/users/overview">Português (Brasil)</a> |
+<a href="https://qwenlm.github.io/qwen-code-docs/ko/users/overview">한국어</a>
 
 </div>