Set detail not details

[khiga8]

I noticed that the lint error message for no-default-alt-text doesn't include the error context like, [For image: Screen Shot 2022-06-26 at 7 41 30 PM] even though it's defined in the rule.

This is because details is being set instead of detail. I fixed this issue and added a test to ensure the error message and error detail is what we expect.

Now the error message that appears as part of the lint error should include the error context:

GH001/no-default-alt-text Images should not use the MacOS default screenshot filename as alternate text (alt text). If you have not changed this file, try merging main with your branch. 
For more information see: https://primer.style/design/accessibility/alternative-text-for-images 
[For image: Screen Shot 2022-06-26 at 7 41 30 PM]

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[smockle]

Good catch; thanks for fixing this! I left one non-blocking comment.