bpo-35086: Fix tkinter example "A Simple Hello World Program". (GH-10160)

The root widget was accessed as a global variable in the Application's method.
(cherry picked from commit a80af77087)

Co-authored-by: Daniel Lovell <lovell.daniel92@gmail.com>

Doc/library/tkinter.rst

@@ -205,6 +205,7 @@ A Simple Hello World Program
     class Application(tk.Frame):
         def __init__(self, master=None):
             super().__init__(master)
+            self.master = master
             self.pack()
             self.create_widgets()
 
@@ -215,7 +216,7 @@ A Simple Hello World Program
             self.hi_there.pack(side="top")
 
             self.quit = tk.Button(self, text="QUIT", fg="red",
-                                  command=root.destroy)
+                                  command=self.master.destroy)
             self.quit.pack(side="bottom")
 
         def say_hi(self):