Fix method name for building associated object in nested_model_forms guide

guides/source/nested_model_forms.md

@@ -106,7 +106,7 @@ Consider the following typical RESTful controller which will prepare a new Perso
 class PeopleController < ApplicationController
   def new
     @person = Person.new
-    @person.built_address
+    @person.build_address
     2.times { @person.projects.build }
   end