Correct typo: "Free" to "Three" in comments (#145689)

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Author: dwzrlp

packages/flutter/lib/src/material/dropdown.dart

@@ -861,7 +861,7 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 /// {@tool dartpad}
 /// This sample shows a [DropdownButton] with a large arrow icon,
 /// purple text style, and bold purple underline, whose value is one of "One",
-/// "Two", "Free", or "Four".
+/// "Two", "Three", or "Four".
 ///
 /// ![](https://flutter.github.io/assets-for-api-docs/assets/material/dropdown_button.png)
 ///