fix: use appropriate variable in error message

[ShabiShett07]

none

Description

What is the purpose of this pull request?

This pull request:

• change M to N wherever it is required.

Related Issues

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This pull request:

• none

Questions

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No.

Other

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Checklist

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• Read, understood, and followed the contributing guidelines.

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[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
blas/base/sgemm	$\color{green}835/835$ $\color{green}+100.00\%$	$\color{green}118/118$ $\color{green}+100.00\%$	$\color{green}8/8$ $\color{green}+100.00\%$	$\color{green}835/835$ $\color{green}+100.00\%$

The above coverage report was generated for the changes in this PR.

[kgryte]

Thanks for catching and fixing, @ShabiShett07!