Minor update to docs/aws.md

[ysim]

Updates the backtick syntax to $(...), plus a minor grammar fix.

I wasn't sure if I needed to fill out a CLA for just a documentation update. Let me know if I should!

Update: Never mind, I've signed the agreement now!

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[chrislovecnm]

/ok-to-test

Thanks a ton!

/lgtm

[KashifSaadat]

Hmm, lgtm didn't register?

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chrislovecnm, KashifSaadat

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[k8s-github-robot]

Automatic merge from submit-queue.