Remove invalid character for UTF-8

Closes Homebrew/homebrew#10962.

Signed-off-by: Max Howell <max@methylblue.com>

Library/Homebrew/cmd/doctor.rb

@@ -220,7 +220,7 @@ def check_cc
       You have no /usr/bin/cc.
       This means you probably can't build *anything*. You need to install the CLI
       Tools for Xcode. You can either download this from http://connect.apple.com/
-      or install them from inside Xcode’s preferences. Homebrew does not require
+      or install them from inside Xcode's preferences. Homebrew does not require
       all of Xcode! You only need the CLI tools package!
     EOS
   end