Fix misspelled author name “Matthew” to “Mathew” in cyclic graphs notebook

In `cyclic_graphs/cyclic_graphs.ipynb`, the introductory text says:

> These independent sets match the best known construction reported by Matthew & Östergård (2016).

The first author's surname is misspelled. The cited paper lists the authors as K. Ashik Mathew and Patric R. J. Östergård.

The text should therefore read:

> These independent sets match the best known construction reported by Mathew & Östergård (2016).

This appears to be a documentation-only typo.

Open `cyclic_graphs/cyclic_graphs.ipynb` and find the introductory text containing the citation to Mathew & Östergård (2016). Correct the author's surname from “Matthew” to “Mathew” and verify that the notebook contains the corrected wording and no unintended changes.

Written by the indexing model from the issue text.