minor symfony#11359 [Upgrade] Fixed markdown syntax (lyrixx)

This PR was merged into the 2.3 branch.

Discussion
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[Upgrade] Fixed markdown syntax

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

Commits
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deb70c7 [Upgrade] Fixed markdown syntax

Author: fabpot

UPGRADE-2.3.md

@@ -110,9 +110,9 @@ Form
    }
    ```
 
- * The *_SET_DATA events are now guaranteed to be fired *after* the children
+ * The `*_SET_DATA` events are now guaranteed to be fired *after* the children
    were added by the FormBuilder (unless setData() is called manually). Before,
-   the *_SET_DATA events were sometimes thrown before adding child forms,
+   the `*_SET_DATA` events were sometimes thrown before adding child forms,
    which made it impossible to remove child forms dynamically.
 
    A consequence of this change is that you need to set the "auto_initialize"