fix url error.

flask_app/flask_app.py

@@ -29,7 +29,7 @@ def hello(name=None):
 def form():
     if request.method == "POST":
         username = request.form.get("username")
-        return redirect(url_for("hello", name=username))  # 提交后跳转到打招呼页面
+        return redirect(url_for("./hello", name=username))  # 提交后跳转到打招呼页面
     return """
         <form method="post">
             <label>输入你的名字：</label>