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

Elis?e 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/3a3a88db1fc2/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.

docs/README-macosx.md

@@ -21,7 +21,7 @@ the gcc-fat.sh script in build-scripts:
 
     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