Fixed the link to Functional Tests

Author: royklutman

testing.rst

@@ -48,7 +48,7 @@ Unit Tests
 
 A unit test is a test against a single PHP class, also called a *unit*. If you
 want to test the overall behavior of your application, see the section about
-`Functional Tests`_.
+:ref:`Functional Tests <functional-tests>`.
 
 Writing Symfony unit tests is no different from writing standard PHPUnit
 unit tests. Suppose, for example, that you have an *incredibly* simple class