Fix spelling

A simple spelling fix, that annoyed me each time I stumble over it.

This is a test if you agree to such changes. Dunno if spelling fixes fall under the "we fix when we touch the file for other reasons" - category.

I'd like to see this backported to at least 2-3-stable. (Want to bring our diff to zero)
In the example body you see `LineItem#adjustments` gets accessed together with a matcher:

`expect(second_line_item.reload.adjustments).to be_empty` to me it makes sense to use the plural form when the code under test also does it.