base64 -D is OSX specific

Looks like we are suggesting base64 -D which is OSX specific, see https://gitlab.com/gitlab-org/gitlab-ce/issues/54603#note_124210624. BSD and Linux both seem to use base64 -d

• https://linux.die.net/man/1/base64
• https://www.freebsd.org/cgi/man.cgi?query=base64&apropos=0&sektion=0&manpath=FreeBSD+12.0-RELEASE+and+Ports&arch=default&format=html
• https://www.unix.com/man-page/osx/1/base64/

I think we should at least mention both, or use --decode !

$ gg "base64 -D"
doc/user/project/clusters/eks_and_gitlab/index.md:       kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 -D
doc/user/project/clusters/index.md:  1. Get the token for the appropriate secret by running: `kubectl get secret <SECRET_NAME> -o jsonpath="{['data']['token']}" | base64 -D`.
doc/user/project/clusters/index.md:- CA certificate, run `kubectl get secret <secret name> -o jsonpath="{['data']['ca\.crt']}" | base64 -D`.
tkgl:gitlab tkuah$ gg "base64 -d"