minor #19085 FormBuilderInterface: fix getForm() return type. (taueres)

This PR was merged into the 2.7 branch.

Discussion
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FormBuilderInterface: fix getForm() return type.

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

FormBuilderInterface->getForm() should depend on abstractions and
not implementations as a return type.

Commits
-------

3fa081c FormBuilderInterface: fix getForm() return type.

src/Symfony/Component/Form/FormBuilderInterface.php

@@ -81,7 +81,7 @@ interface FormBuilderInterface extends \Traversable, \Countable, FormConfigBuild
     /**
      * Creates the form.
      *
-     * @return Form The form
+     * @return FormInterface The form
      */
     public function getForm();
 }