Documentation bug fixing

[YANGMAO-ZHANG]

Correspond to the issue #85
Which format of command would be better?
kubectl exec -ti utils -n default -- /bin/sh
or
kubectl exec -ti utils -n <utils-namespace> -- /bin/sh

[CLAassistant]

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[benjaminhuo]

@YANGMAO-ZHANG Please sign the CLA and sign your commit like below:

git commit -s --amend
git push -f