minor #11670 fixes missing "use" declaration for the ClockMock (chdeliens)

This PR was submitted for the 4.3 branch but it was merged into the 3.4 branch instead (closes #11670).

Discussion
----------

fixes missing "use" declaration for the ClockMock

so that one can reuse that example out of the box.
may be wise to also patch older maintained branches to guarantee consistency in the docs.

Commits
-------

dce4934 fixes missing "use" declaration for the ClockMock

Author: javiereguiluz

components/phpunit_bridge.rst

@@ -378,6 +378,7 @@ different class, do it explicitly using ``ClockMock::register(MyClass::class)``:
 
     use App\MyClass;
     use PHPUnit\Framework\TestCase;
+    use Symfony\Bridge\PhpUnit\ClockMock; 
 
     /**
      * @group time-sensitive