Update README.rst for Python 3 on macOS

README.rst

@@ -86,16 +86,16 @@ To create binaries, create the 'packages' directory::
 
 This directory contains the python dependencies used by Electrum.
 
-Mac OS X
+Mac OS X / macOS
 --------
 
 ::
 
     # On MacPorts installs: 
-    sudo python setup-release.py py2app
+    sudo python3 setup-release.py py2app
     
     # On Homebrew installs: 
-    ARCHFLAGS="-arch i386 -arch x86_64" sudo python setup-release.py py2app --includes sip
+    ARCHFLAGS="-arch i386 -arch x86_64" sudo python3 setup-release.py py2app --includes sip
     
     sudo hdiutil create -fs HFS+ -volname "Electrum" -srcfolder dist/Electrum.app dist/electrum-VERSION-macosx.dmg