fix a potential bounds error

[andrewjradcliffe]

Truly, very minor. I happened upon this when searching for @turbo
uses.

[chriselrod]

Thanks, definitely shouldn't be using 1:length(C).

[codecov]

Codecov Report

Merging #148 (7817fd0) into master (ad7f099) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   88.06%   88.06%           
=======================================
  Files          13       13           
  Lines         930      930           
=======================================
  Hits          819      819           
  Misses        111      111           

Impacted Files	Coverage Δ
src/matmul.jl	91.84% <100.00%> (ø)

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