minor #8013 correct PHP code (MedUnes, xabbuh)

This PR was submitted for the 3.3 branch but it was merged into the 2.8 branch instead (closes #8013).

Discussion
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correct PHP code

Commits
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9d1eab7 [#8013] revert wrong data provider change
80e664a correct PHP code

Author: xabbuh

form/unit_testing.rst

@@ -196,7 +196,7 @@ allows you to return a list of extensions to register::
             $this->validator
                 ->method('validate')
                 ->will($this->returnValue(new ConstraintViolationList()));
-            $validator
+            $this->validator
                 ->method('getMetadataFor')
                 ->will($this->returnValue(new ClassMetadata(Form::class)));