brew doctor - remove double negative

Library/Homebrew/brew_doctor.rb

@@ -236,8 +236,8 @@ end
 def check_homebrew_prefix
   unless HOMEBREW_PREFIX.to_s == '/usr/local'
     puts <<-EOS.undent
-      You can install Homebrew anywhere you want, but some brews may not work
-      correctly if you're not installing to /usr/local.
+      You can install Homebrew anywhere you want, but some brews may only work
+      correctly if you install to /usr/local.
 
     EOS
   end