Update matmul.jl

[thimotedupuch]

More idiomatic way of handling of indices

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (8d44613) to head (d2017e9).
⚠️ Report is 10 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (8d44613) and HEAD (d2017e9). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (8d44613)	HEAD (d2017e9)
	20	10

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #646       +/-   ##
==========================================
- Coverage   51.24%   0.00%   -51.25%     
==========================================
  Files          21      20        -1     
  Lines        1610    1462      -148     
==========================================
- Hits          825       0      -825     
- Misses        785    1462      +677     

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[christiangnrd]

Thanks!