update `api.Allocations.Stop` signature to use `WriteOptions`

[MattJustMatt]

https://github.com/hashicorp/nomad/blob/290a310544f70a9727643eefb856f6d5d0150fdf/api/allocations.go#L189

The docs reference a change in the function signature for stopping allocations starting in Nomad 1.6.0-- I'm on 1.6.0 and the old signature is still used.