fix typo in readme

[axsk]

its a 6x6 matrix (with a length 5 subdiagonal)

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (7f123af) 89.43% compared to head (3ccc8b9) 89.44%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   89.43%   89.44%   +0.01%     
==========================================
  Files          25       25              
  Lines        3510     3516       +6     
==========================================
+ Hits         3139     3145       +6     
  Misses        371      371              

Files Changed	Coverage Δ
src/generic/matmul.jl	81.20% <100.00%> (+0.78%)	⬆️
src/lapack.jl	89.09% <100.00%> (ø)

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[jishnub]

Thanks!