prev-label props for multiStep form should be name previous-label

[thriist]

Hi there, In the doc (https://github.com/formkit/docs-content/blob/master/plugins/multi-step.md), to customise the label of the previous button of a multiStep form the props is named prev-label.

As i tried to use it, i notice that in "packages/addons/src/plugins/multiStep/sections/stepPrevious.ts" the props name is "previous-label"

that's it, i can make the PR if you want but it would be my first on a public rep and it is a bit scarry ^^'

Regards

[andrew-boyd]

Go for it! Will happily merge a PR for this — get you that contributor credit on the repo. 😄