upper case restriction doesn't exist

During my test, the restriction of "form of DNS labels" doesn't exist, we can create labels with upper-case letters.

docs/concepts/configuration/assign-pod-node.md

@@ -40,8 +40,6 @@ Run `kubectl get nodes` to get the names of your cluster's nodes. Pick out the o
 
 If this fails with an "invalid command" error, you're likely using an older version of kubectl that doesn't have the `label` command. In that case, see the [previous version](https://github.com/kubernetes/kubernetes/blob/a053dbc313572ed60d89dae9821ecab8bfd676dc/examples/node-selection/README.md) of this guide for instructions on how to manually set labels on a node.
 
-Also, note that label keys must be in the form of DNS labels (as described in the [identifiers doc](https://git.k8s.io/community/contributors/design-proposals/architecture/identifiers.md)), meaning that they are not allowed to contain any upper-case letters.
-
 You can verify that it worked by re-running `kubectl get nodes --show-labels` and checking that the node now has a label.
 
 ### Step Two: Add a nodeSelector field to your pod configuration