Tech writer edits to nginx-cert-gke.md (#16)

Hi there @mkysel 👋 - as kindly requested - here are some suggested tech
writer 🤓 edits to nginx-cert-gke.md. As discussed, I did not perform the
steps myself - but just went through and found places where we might add
some tech writer techniques to help with flow.

All just suggestions - feel free to take only what you think works!

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Co-authored-by: Martin Kysel <[email protected]>