readme: correct webpage URL to use HTTPS.
diff --git a/README.txt b/README.txt
index 84c335c..e8630c7 100644
--- a/README.txt
+++ b/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