docs: adjust kubectl_path_documents example

[johnknutsonhc]

The provider documentation calls out the manifests attribute as being well-suited for for_each expressions. The for_each example, however, uses the documents attribute instead.

This PR updates the for_each example to use the manifests attribute.

This PR addresses https://github.com/gavinbunney/terraform-provider-kubectl/issues/286

[gavinbunney]

Thanks!