Correct version drift in cluster-upgrade.md

Correct the description of version drift during upgrade to match
the current kubernetes documentation. Node version should not be
newer than the cluster version.

Author: larvacea

userdocs/src/usage/cluster-upgrade.md

@@ -12,11 +12,11 @@ An _`eksctl`-managed_ cluster can be upgraded in 3 easy steps:
 Please make sure to read this section in full before you proceed.
 
 ???+ info
-    Kubernetes supports version drift of up-to two minor versions during upgrade
-    process. So nodes can be up to two minor versions ahead or behind the control plane
+    Kubernetes supports version drift of up to two minor versions during the upgrade
+    process. Nodes can be up to two minor versions behind, but never ahead of the control plane
     version. You can only upgrade the control plane one minor version at a time, but
-    nodes can be upgraded more than one minor version at a time, provided the nodes stay
-    within two minor versions of the control plane.
+    nodes can be upgraded more than one minor version at a time, provided their version
+    does not become greater than the control plane version.
 
 ???+ info
     The old `eksctl update cluster` will be deprecated. Use `eksctl upgrade cluster` instead.