minor title update in gui

learning_python3_gui.md

@@ -55,6 +55,8 @@ if __name__ == "__main__":
     app.mainloop()
 ```
 
+## Adding widgets
+
 We can add content to this window, such as labels, input boxes and buttons as follows.
 
 

readme.md

@@ -107,6 +107,7 @@ Table of Contents
 
 * [tkinter](./learning_python3_gui.md#tkinter)
    * [Tkinter helloworld](./learning_python3_gui.md#tkinter-helloworld)
+   * [Adding widgets](./learning_python3_gui.md#adding-widgets)
    * [Try it](./learning_python3_gui.md#-try-it)
 * [Coding challenge - Guess the number](./learning_python3_gui.md#coding-challenge---guess-the-number)
    * [MVC design pattern](./learning_python3_gui.md#mvc-design-pattern)