Fix base64 example to use universal option

`-D` is OSX speific and is invalid on Linux, etc

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/55218

doc/user/project/clusters/eks_and_gitlab/index.md

@@ -49,7 +49,7 @@ A few details from the EKS cluster will be required to connect it to GitLab:
     -  Get the certificate with:
 
        ```sh
-       kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 -D
+       kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 --decode
        ```
 
 1.  **Create admin token**: A `cluster-admin` token is required to install and

doc/user/project/clusters/index.md

@@ -157,8 +157,8 @@ To determine the:
 - API URL, run `kubectl cluster-info | grep 'Kubernetes master' | awk '/http/ {print $NF}'`.
 - Token:
   1. List the secrets by running: `kubectl get secrets`. Note the name of the secret you need the token for.
-  1. Get the token for the appropriate secret by running: `kubectl get secret <SECRET_NAME> -o jsonpath="{['data']['token']}" | base64 -D`.
-- CA certificate, run `kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 -D`.
+  1. Get the token for the appropriate secret by running: `kubectl get secret <SECRET_NAME> -o jsonpath="{['data']['token']}" | base64 --decode`.
+- CA certificate, run `kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 --decode`.
 
 ## Security implications