docs: fix broken link to k8s pod specs
In a :ref:`Determined cluster running on Kubernetes <determined-on-kubernetes>`, tasks (e.g.,
experiments, notebooks) are executed by launching one or more Kubernetes pods. Users can customize
these pods by providing custom `pod specs
-<https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.18/#pod-v1-core>`__. Common use
+<https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.19/#pod-v1-core>`__. Common use
cases include assigning pods to specific nodes, specifying additional volume mounts, and attaching
permissions. Configuring pod specs is not required to use Determined on Kubernetes.
