set_text_hidden function of UITextEntryLine does not hide already entered symbols
**Describe the bug**
When calling set_text_hidden(True) on a non-empty UITextEntryLine component it will not update already entered text to become hidden, instead only newly entered symbols will be hidden. 

**To Reproduce**
Steps to reproduce the behaviour:
1. Add a new UITextEntryLine component to the UIManager
2. Create a button which executes set_text_hidden(True)
3. Enter text in UITextEntryLine
4. Press button to hide text

**Expected behaviour**
I expect already typed text to become hidden by displaying the '●' character instead.  In reality only newly typed characters will become hidden. 

**Screenshots**
[Example](https://i.gyazo.com/e7f6284b4e5d3699d3eab92cf57bceff.mp4)
Button code:
     
             for event in pygame.event.get():
             
                 if event.type == pygame_gui.UI_BUTTON_PRESSED:                     
                 
                     if event.ui_element == self.visibility_button:
                     
                        self.visibility_button.disable()
                        self.visibility_button.hide()
                        self.hidden_button.enable()
                        self.hidden_button.show()
                        self.input_password.set_text_hidden(False)
                                                
                    if event.ui_element == self.hidden_button:
                    
                        self.hidden_button.disable()
                        self.hidden_button.hide()
                        self.visibility_button.enable()
                        self.visibility_button.show()
                        self.input_password.set_text_hidden(True)


**Platform and software:**
 - OS:  Windows
 - Pygame GUI version 0.6.8
 - Pygame version 2.1.2



diff --git a/pygame_gui/elements/ui_text_entry_line.py b/pygame_gui/elements/ui_text_entry_line.py
index e507dd9..7efa44c 100644
--- a/pygame_gui/elements/ui_text_entry_line.py
+++ b/pygame_gui/elements/ui_text_entry_line.py
@@ -200,7 +200,7 @@ class UITextEntryLine(UIElement):
                         raise ValueError('Selected font for UITextEntryLine does not contain '
                                          '●, * or . characters used for hidden text. Please choose'
                                          'a different font for this element')
-                display_text = self.hidden_text_char*len(self.text)
+            display_text = self.hidden_text_char*len(self.text)
 
         theming_parameters = {'normal_bg': self.background_colour,
                               'normal_text': self.text_colour,
['pygame_gui/elements/ui_text_entry_line.py']