minor symfony#2758 Add missing DefaultActionTrait in a LiveComponent code example (welcoMattic)

This PR was merged into the 2.x branch.

Discussion
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Add missing DefaultActionTrait in a LiveComponent code example

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Docs?         | yes<!-- required for new features -->
| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT

To let users able to copy/paste this code and make it works out of the box.

Commits
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e3efb61 Update index.rst

Author: Kocal

src/LiveComponent/doc/index.rst

@@ -208,6 +208,8 @@ Let's make our component more flexible by adding a ``$max`` property::
     #[AsLiveComponent]
     class RandomNumber
     {
+        use DefaultActionTrait;
+
         #[LiveProp]
         public int $max = 1000;