chore: fix typo "eighter" -> "either"

[shenwei356]

Thanks for this. It seems that you're using a markdown linter, which changed a lot of content. I'm sorry I won't merge this. I might just replace the wrong word, which appears at four places.

$ rg eighter
README.md
274:8. If somes fields have only a double-quote eighter in the beginning or in the end, e.g.,

csvtk/cmd/root.go
61:  8. If somes fields have only a double-quote eighter in the beginning or in the end, e.g.,

doc/docs/usage.md
27:8. If somes fields have only a double-quote eighter in the beginning or in the end, e.g.,
151:  8. If somes fields have only a double-quote eighter in the beginning or in the end, e.g.,

Damn it, somes should be some.