Fix BFS distance notation typo in Day 13 graph notes

[Nullkernel]

Motivation

• Correct a typographical mistake in the BFS documentation where “distance kkk” was written instead of the intended level notation k to keep the notes clear and consistent.

Description

• Updated the BFS section of Notes/Day-13-graphs.md to change “distance kkk” to “distance k” and performed a quick proofreading of the nearby BFS text for similar notation issues.

Testing

• No runtime tests required for this documentation-only change and the update was verified with git diff -- Notes/Day-13-graphs.md and by printing the relevant lines with nl -ba Notes/Day-13-graphs.md | sed -n '18,32p', and the commit completed successfully.

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Codex Task