readme: correct webpage URL to use HTTPS.

README.txt

@@ -6,7 +6,7 @@
                                 Version 2.0
 
 ---
-http://www.libsdl.org/
+https://www.libsdl.org/
 
 Simple DirectMedia Layer is a cross-platform development library designed
 to provide low level access to audio, keyboard, mouse, joystick, and graphics