doctor: minor grammatical fix

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -752,7 +752,7 @@ end
 def check_git_status
   status_cmd = "git --git-dir=#{HOMEBREW_REPOSITORY}/.git --work-tree=#{HOMEBREW_PREFIX} status -s #{HOMEBREW_PREFIX}/Library/Homebrew"
   if system "/usr/bin/which -s git" and File.directory? HOMEBREW_REPOSITORY+'.git' and not `#{status_cmd}`.empty?
-    ohai "You have uncommitted modifications to Homebrew core"
+    ohai "You have uncommitted modifications to Homebrew's core."
     puts "Unless you know what you are doing, you should: git reset --hard"
     puts
   end