Revert "Update wiki.libsdl.org urls to libsdl-org/SDL/wiki (#4069)"

This reverts commit 82a96afa70.

We ended up not moving to GitHub's wiki after all and built
https://github.com/icculus/ghwikipp to fill the gap...since the wiki is
back at https://wiki.libsdl.org/, put back the original URLs.

docs/README-platforms.md

@@ -4,5 +4,5 @@ Platforms
 We maintain the list of supported platforms on our wiki now, and how to
 build and install SDL for those platforms:
 
-    https://github.com/libsdl-org/SDL/wiki/Installation
+    https://wiki.libsdl.org/Installation