Fixed bug 3460 - docs/README-macosx.md: g++fat.sh should be g++-fat.sh in universal build command

Elisée Maurer

I scratched my head for a while until I realized there's a typo in the command listed in the instructions for universal Mac builds: https://hg.libsdl.org/SDL/file/c1bb718f6c3f/docs/README-macosx.md#l24

It should say `g++-fat.sh` but instead it says `g++fat.sh`, which makes `./configure` fail with a C++ preprocessor error.
diff --git a/docs/README-macosx.md b/docs/README-macosx.md
index 34234b1..869de2e 100644
--- a/docs/README-macosx.md
+++ b/docs/README-macosx.md
@@ -21,7 +21,7 @@
 

     mkdir mybuild

     cd mybuild

-    CC=$PWD/../build-scripts/gcc-fat.sh CXX=$PWD/../build-scripts/g++fat.sh ../configure

+    CC=$PWD/../build-scripts/gcc-fat.sh CXX=$PWD/../build-scripts/g++-fat.sh ../configure

     make

     sudo make install