Fix mypy errors at bfs_zero_one_shortest_path

[mhihasan]

Describe your change:

• Fix a bug or typo in an existing algorithm?

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.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

Related issue #4052