chore: minor improvement for docs

[ginavalent]

minor improvement for docs

[cursor]

PR Summary

Low Risk
Comment-only edits in example files with no runtime or API impact.

Overview
Documentation-only tweaks in three example Cairo files: doc comments in fib.cairo and fib_loop.cairo now say nth instead of n'th, and the module-nesting comment in fib_unary.cairo uses two levels (plural) instead of two level. No code or behavior changes.

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This change is 

[orizi]

@orizi reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ginavalent).