[docs] broken link on `kubernetes_manifest`

[cwegener]

link for yamldecode() is broken.

https://github.com/hashicorp/terraform-provider-kubernetes/blob/e1f7cfdf31809c81f3bf9d8d8d0ec3322d418f15/docs/resources/manifest.md#L10

current broken target:

https://www.terraform.io/docs/configuration/functions/yamldecode.html

actual working target:

https://developer.hashicorp.com/terraform/language/functions/yamldecode

(and also, what is up with the unlimited textwidth of markdown files in this repo ...)