minor #13744 Fix form unit testing code sample (yoannrenard)

This PR was merged into the 3.4 branch.

Discussion
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Fix form unit testing code sample

Quick fix of this little bit of code that was misleading.

Commits
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62aee7c Fix form unit testing code sample

Author: javiereguiluz

form/unit_testing.rst

@@ -54,9 +54,9 @@ The simplest ``TypeTestCase`` implementation looks like the following::
                 'test2' => 'test2',
             ];
 
-            $formData = new TestObject();
+            $model = new TestObject();
             // $formData will retrieve data from the form submission; pass it as the second argument
-            $form = $this->factory->create(TestedType::class, $formData);
+            $form = $this->factory->create(TestedType::class, $model);
 
             $expected = new TestObject();
             // ...populate $object properties with the data stored in $formData
@@ -68,7 +68,7 @@ The simplest ``TypeTestCase`` implementation looks like the following::
             $this->assertTrue($form->isSynchronized());
 
             // check that $formData was modified as expected when the form was submitted
-            $this->assertEquals($expected, $formData);
+            $this->assertEquals($expected, $model);
         }
 
         public function testCustomFormView()