minor symfony#6925 Method "$this->getMock()" is deprecated (JohnnyEvo)

This PR was submitted for the 3.1 branch but it was merged into the 2.8 branch instead (closes symfony#6925).

Discussion
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Method "$this->getMock()" is deprecated

Commits
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e151754 Method "$this->getMock()" is depreciated

Author: weaverryan

form/unit_testing.rst

@@ -132,7 +132,7 @@ make sure the ``FormRegistry`` uses the created instance::
         protected function setUp()
         {
             // mock any dependencies
-            $this->entityManager = $this->getMock('Doctrine\Common\Persistence\ObjectManager');
+            $this->entityManager = $this->getMockBuilder('Doctrine\Common\Persistence\ObjectManager')->getMock();
 
             parent::setUp();
         }