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-git.md

@@ -15,5 +15,5 @@ If you are building SDL via configure, you will need to run autogen.sh
 before running configure.
 
 There is a web interface to the Git repository at:
-	https://github.com/libsdl-org/SDL/
+	http://github.com/libsdl-org/SDL/
 

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
 

docs/README.md

@@ -27,7 +27,7 @@ The best way to learn how to use SDL is to check out the header files in
 the "include" subdirectory and the programs in the "test" subdirectory.
 The header files and test programs are well commented and always up to date.
 
-More documentation and FAQs are available online at [the wiki](https://github.com/libsdl-org/SDL/wiki/)
+More documentation and FAQs are available online at [the wiki](http://wiki.libsdl.org/)
 
 - [Android](README-android.md)
 - [CMake](README-cmake.md)