'brew doctor' provides bad example command

If there are multiple dependencies missing, 'brew doctor' can suggest
that you run something like:

  brew install autoconf libyaml autoconf, libyaml

This commit improves that output.

Closes Homebrew/homebrew#10720.

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

Library/Contributions/examples/brew-missing.rb

@@ -29,7 +29,7 @@ def main
 
     unless missing_deps.empty?
       print "#{f.name}: " if formulae_to_check.size > 1
-      puts "#{missing_deps * ', '}"
+      puts "#{missing_deps * ' '}"
     end
   end
 end

Library/Homebrew/cmd/doctor.rb

@@ -718,7 +718,9 @@ def check_missing_deps
   s = []
   `brew missing`.each_line do |line|
     line =~ /(.*): (.*)/
-    s << $2 unless s.include? $2
+    $2.split.each do |dep|
+        s << dep unless s.include? dep
+    end
   end
   if s.length > 0 then <<-EOS.undent
     Some installed formula are missing dependencies.