Fix broken link to Fixtures documentation on guides

and activerecord tests

[ci skip]

activerecord/test/fixtures/tasks.yml

-# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/Fixtures.html
+# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
 first_task:
   id: 1
   starting: 2005-03-30t06:30:00.00+01:00

guides/code/getting_started/test/fixtures/comments.yml

-# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/Fixtures.html
+# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
 
 one:
   commenter: MyString

guides/code/getting_started/test/fixtures/posts.yml

-# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/Fixtures.html
+# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
 
 one:
   title: MyString