Fix incorrect wording with secrets
Fix incorrect wording with secrets

1) The example given in the section "Using Secrets as Environment
Variables" incorrectly stated the secret was using a volume.

2) The commands used to produce secrets in the section "Use-Case: Pods
with prod / test credentials" has been modified such that the literal
used matches the text for what expected files should be on the
filesystem (changed user to username).

erictune/erictune.github.io,erictune/erictune.github.io,erictune/erictune.github.io,aidevops/kubernetes.github.io,rata/kubernetes.github.io,rata/kubernetes.github.io,aidevops/kubernetes.github.io,rata/kubernetes.github.io,aidevops/kubernetes.github.io,aidevops/kubernetes.github.io,aidevops/kubernetes.github.io,erictune/erictune.github.io,rata/kubernetes.github.io,erictune/erictune.github.io,rata/kubernetes.github.io