minor #10352 [Fix] Fixed the link to Functional Tests (royklutman)

This PR was merged into the 2.8 branch.

Discussion
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[Fix] Fixed the link to Functional Tests

Fixed a link to Functional Tests, was referring to `#id1` instead of  `#functional-tests`. See the Functional Tests link in the following paragraph: https://symfony.com/doc/current/testing.html#unit-tests

Commits
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daef32d Fixed the link to Functional Tests

Author: javiereguiluz

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