Update Armstrong_number.dart

[nandhakrishna10]

Welcome to Dart community

Describe your change:

• [ ] Add an algorithm?
• [x] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?
• [ ] Add/Update/Fix test cases.

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new Dart files are placed inside an existing directory.
• [x] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[nandhakrishna10]

I fixed this error:

Launching lib\main.dart on Chrome in debug mode... lib/main.dart:10:15: Error: A value of type 'num' can't be assigned to a variable of type 'int'. sum = sum + pow(r, d); ^