[Testing guide] Using "Fixtures API" consistently

- In guides, "API" is used consistently instead of "api".
- Reason behind replacing "fixtures API" to "Fixtures API" is because
  here we are referring to the concept of "fixtures". In other places in
  the guide "fixture" OR "fixtures" is referred to the single or
  multiple instances of test data.
- This is also followup of https://github.com/rails/docrails/commit/aa9d1332b20305dd860b21a7ada06c5252bebad5#commitcomment-8497376.

guides/source/testing.md

@@ -100,7 +100,7 @@ Note: For associations to reference one another by name, you cannot specify the
  attribute on the fixtures. Rails will auto assign a primary key to be consistent between
  runs. If you manually specify an `id:` attribute, this behavior will not work. For more
   information on this association behavior please read the
-  [fixture api documentation](http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html).
+  [Fixtures API documentation](http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html).
 
 #### ERB'in It Up