minor symfony#20431 Update UPGRADE-2.7.md (Aliance)

This PR was merged into the 2.7 branch.

Discussion
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Update UPGRADE-2.7.md

See [Symfony/Component/Form/AbstractTypeExtension.php](https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Form/AbstractTypeExtension.php)

| 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        | –

Commits
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1c0f572 Update UPGRADE-2.7.md

Author: fabpot

UPGRADE-2.7.md

@@ -40,7 +40,7 @@ Form
 ----
 
  * In form types and extension overriding the "setDefaultOptions" of the
-   AbstractType or AbstractExtensionType has been deprecated in favor of
+   AbstractType or AbstractTypeExtension has been deprecated in favor of
    overriding the new "configureOptions" method.
 
    The method "setDefaultOptions(OptionsResolverInterface $resolver)" will