Fix incorrect validation message for missing store credit category

[johnpitchko]

Summary

Fix for #4468. Corrected text in en.yml. Note this is branched off PR #4481, but can rebase of master if you want to merge this one before that one.

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[jarednorman]

Looks good, if you don't mind rebasing out the extra commit from your other PR.

[gsmendoza]

I was able to confirm that the fix works in https://github.com/gsmendoza/solidus-4468/tree/johnpitchko/fix-incorrect-validation-message-for-missing-store-credit-category-id.

[gsmendoza]

Hi @johnpitchko ! @waiting-for-dev and I weren't able to reproduce #4479. Like what @jarednorman said, can you remove the Fix bad MySQL package in Dockerfile commit from this PR? We'll need to investigate that separately. Thanks!

[waiting-for-dev]

Thanks, @johnpitchko. Yeah, we should merge it once the extra commit is removed.

[johnpitchko]

Thanks for your patience @jarednorman @gsmendoza @waiting-for-dev - was out of town for a few days. I dropped the commit so this should be ready now.

[waiting-for-dev]

Thanks, @johnpitchko!