Fix cgi/core code example missing comma

ref https://github.com/rurema/doctree/pull/448
[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59469 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/cgi/core.rb

@@ -153,7 +153,7 @@ class CGI
   #               "language"   => "ja",
   #               "expires"    => Time.now + 30,
   #               "cookie"     => [cookie1, cookie2],
-  #               "my_header1" => "my_value"
+  #               "my_header1" => "my_value",
   #               "my_header2" => "my_value")
   #
   # This method does not perform charset conversion.