Fixing broken link to Certificate Management with kubeadm

A missing '/' on the link will lead to a 404 error. Fixing it.

Author: raonitimo

content/en/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade.md

@@ -148,7 +148,7 @@ Find the latest stable 1.18 version:
 {{< note >}}
 `kubeadm upgrade` also automatically renews the certificates that it manages on this node.
 To opt-out of certificate renewal the flag `--certificate-renewal=false` can be used.
-For more information see the [certificate management guide](/docs/tasks/administer-cluster/kubeadmkubeadm-certs).
+For more information see the [certificate management guide](/docs/tasks/administer-cluster/kubeadm/kubeadm-certs).
 {{</ note >}}
 
 -  Choose a version to upgrade to, and run the appropriate command. For example:
@@ -441,4 +441,4 @@ and post-upgrade manifest file for a certain component, a backup file for it wil
 `kubeadm upgrade node` does the following on worker nodes:
 
 - Fetches the kubeadm `ClusterConfiguration` from the cluster.
-- Upgrades the kubelet configuration for this node.
+- Upgrades the kubelet configuration for this node.