minor #5043 Switched the first example to a static constructor method…

… (kix)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #5043).

Discussion
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Switched the first example to a static constructor method

I believe having a non-static method for class instantiation is not something the documentation should encourage. It's much better to use a named constructor here (also note that `TestObject` class code isn't supplied).

Commits
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118513b Switched the first example to a static constructor method

cookbook/form/unit_testing.rst

@@ -55,8 +55,7 @@ The simplest ``TypeTestCase`` implementation looks like the following::
             $type = new TestedType();
             $form = $this->factory->create($type);
 
-            $object = new TestObject();
-            $object->fromArray($formData);
+            $object = TestObject::fromArray($formData);
 
             // submit the data to the form directly
             $form->submit($formData);