r: Fix Framework link command in caveats

`sudo` is require to create symlinks in `/Library/Frameworks`.

Fixes Homebrew/homebrew#15212.

Formula/r.rb

@@ -64,7 +64,7 @@ class R < Formula
 
     To use this Framework with IDEs such as RStudio, it must be linked
     to the standard OS X location:
-      ln -s "#{prefix}/R.framework" /Library/Frameworks
+      sudo ln -s "#{prefix}/R.framework" /Library/Frameworks
 
     To enable rJava support, run the following command:
       R CMD javareconf JAVA_CPPFLAGS=-I/System/Library/Frameworks/JavaVM.framework/Headers