graphs/BFS: use more sensible graph representation

[nils-werner]

Describe your change:

graphs/BFS uses a very strange text representation of the graph:

g = Graph()
g.add_edge(0, 1)
g.add_edge(0, 2)

i.e.

flowchart LR
    0 --> 1
    0 --> 2

is represented as

0  :  1 -> 2

but I think the format

0  ->  1, 2

is much more understandable

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

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 Python files are placed inside an existing directory.
• [x] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [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}.