Feedback: Manage Redis Enterprise cluster (REC) credentials

[markotrapani]

Page https://redis.io/docs/latest/operate/kubernetes/security/manage-rec-credentials/#change-both-the-rec-username-and-password

In Step 4.1 it says to save the "existing username," to a file, but I think it should be the new username along with the new password, because this is what we are updating the secret value with after creating a new admin user in the UI.

Step 4.3 also appears to have an issue with the formatting of the multiline kubectl command
This is the command I eventually used to update my REC secret that worked:

kubectl create secret generic my-rec \
  --save-config \
  --dry-run=client \
  --from-file=./username --from-file=./password \
  -o yaml | \
kubectl apply -f -

[kaitlynmichael]

@markotrapani please review the changes here: #1108