wxGUI_layer_manager manual: Fix filename for PNG file. (#2052)

gui/wxpython/docs/wxGUI.html

@@ -36,7 +36,7 @@ top of the screen.
 
 <center>
 <br>
-<img src="wxGUI_layer_manager.jpg" border="1" alt="screenshot"><br><br>
+<img src="wxGUI_layer_manager.png" border="1" alt="screenshot"><br><br>
 Figure: Layer Manager screenshot on Ubuntu
 </center>