[Testing guide] Use "Fixtures API" instead of "fixture api".

guides/source/testing.md

@@ -51,7 +51,7 @@ The `test_helper.rb` file holds the default configuration for your tests.
 
 For good tests, you'll need to give some thought to setting up test data.
 In Rails, you can handle this by defining and customizing fixtures.
-You can find comprehensive documentation in the [fixture api documentation](http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html).
+You can find comprehensive documentation in the [Fixtures API documentation](http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html).
 
 #### What Are Fixtures?