更新 SQLAlchemy 中文文档链接

Update broken page URL for SQLAlchemy.

README.md

@@ -639,7 +639,7 @@
     * [Flask之旅](http://spacewander.github.io/explore-flask-zh/)
     * [Flask 扩展文档汇总](https://www.gitbook.com/book/wizardforcel/flask-extension-docs/details)
     * [Flask 大型教程](http://www.pythondoc.com/flask-mega-tutorial/index.html)
-    * [SQLAlchemy 中文文档](https://github.com/sixu05202004/sqlalchemy-docs-cn)
+    * [SQLAlchemy 中文文档](http://docs.jinkan.org/docs/flask-sqlalchemy/)
 * web.py
     * [web.py 0.3 新手指南](http://webpy.org/tutorial3.zh-cn)
     * [Web.py Cookbook 简体中文版](http://webpy.org/cookbook/index.zh-cn)