Update arc_length.py

[Guduly]

Wrote the output of testcase

Describe your change:

Added a comment to show output of the testcase.
Please tell me if I made an error, thank you.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
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• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[tianyizheng02]

Instead of adding the expected output as a comment, could you add this as a test in the docstring?

>>> arc_length(90, 10)
15.7079632679

That way, we can use unit testing to ensure that the output is actually correct.

[Guduly]

Changes made