requirement: update cask wording

We now have "magic" to automatically handle `brew cask` and `brew bundle`
calls without needing to tap those taps manually beforehand. Let's reflect that
in this wording for consistency.

Closes #427.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/Homebrew/requirement.rb

@@ -33,10 +33,10 @@ class Requirement
   def message
     s = ""
     if cask
-      s +=  <<-EOS.undent
+      s += <<-EOS.undent
 
         You can install with Homebrew Cask:
-          brew install Caskroom/cask/#{cask}
+          brew cask install #{cask}
       EOS
     end