Update doc/user/content/self-managed-deployments/upgrading/upgrade-on-kind.md

Co-authored-by: val-materialize <valerie.chiang@materialize.com>

Author: kay-kim

doc/user/content/self-managed-deployments/upgrading/upgrade-on-kind.md

@@ -43,7 +43,7 @@ reference](https://kubernetes.io/docs/reference/kubectl/quick-reference/).
 {{< important >}}
 
 The following procedure performs a rolling upgrade, where both the old and new
-Materialize instances are running before the old instance are removed.
+Materialize instances are running before the old instances are removed.
 When performing a rolling upgrade, ensure you have enough resources to support
 having both the old and new Materialize instances running.