Fix minor typo in comments

[CynaCons]

Description

This PR fixes a minor typo in a comment.

Notes: "flutter analyze" failed with following result:

Running "flutter pub get" in flutter_simple_treeview...          2,933ms
Analyzing flutter_simple_treeview...                                    

  error - Target of URI doesn't exist: 'package:url_launcher/url_launcher.dart' - example\lib\main.dart:8:8 - uri_does_not_exist
  error - The method 'launchUrl' isn't defined for the type 'Demo' - example\lib\main.dart:32:48 - undefined_method

2 issues found. (ran in 18.6s)

Related Issues

Not Applicable.

Checklist

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• I signed the CLA.
• All tests from running flutter test pass.
• flutter analyze does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.