minor symfony#24939 fixed typo (mickaelandrieu)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#24939).

Discussion
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fixed typo

| Q             | A
| ------------- | ---
| Branch?       | master / 3.4?
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        |  it's actually part of docs <!--highly recommended for new features-->

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Commits
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bfede8c fixed typo

Author: nicolas-grekas

UPGRADE-3.4.md

@@ -116,7 +116,7 @@ Form
    ```php
    class MyTimezoneType extends AbstractType
    {
-       public function. getParent()
+       public function getParent()
        {
            return TimezoneType::class;
        }