grains: use $v1 instead of $10 as output register for the high word

[danilopiazza]

The grains exercise instructions suggest using the $10 register to store the high word of the output, but the test runner expects it in $v1.

[github-actions]

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[keiravillekode]

lgtm

[danilopiazza]

lgtm

Thanks! Should I post the link to this PR on the relevant forum topic?

[keiravillekode]

Yes.

[danilopiazza]

Yes.

Done, thank you! https://forum.exercism.org/t/grains-exercise-typo/8173

[keiravillekode]

resolves #198