Update deprecated activeColor to activeThumbColor in Learn Flutter example code (#12691)

_Description of what this PR is changing or adding, and why:_
Replaces the deprecated `activeColor` property with `activeThumbColor`
in Learn Flutter example code
(https://docs.flutter.dev/get-started/fundamentals/user-input#switch) to
help new Flutter developers learn current best practices and avoid using
deprecated APIs.

_Issues fixed by this PR (if any):_
#12690 

_PRs or commits this PR depends on (if any):_

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Author: miinhho

src/content/get-started/fundamentals/user-input.md

@@ -725,7 +725,7 @@ Widget build(BuildContext context) {
   return Switch(
     // This bool value toggles the switch.
     value: light,
-    activeColor: Colors.red,
+    activeThumbColor: Colors.red,
     onChanged: (bool value) {
       // This is called when the user toggles the switch.
       setState(() {