Fix typo: `has_many :pets`, not `pets :has_many`.

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -281,7 +281,7 @@ def create!(attributes = {}, &block)
       # so method calls may be chained.
       #
       #   class Person < ActiveRecord::Base
-      #     pets :has_many
+      #     has_many :pets
       #   end
       #
       #   person.pets.size # => 0