Updates aws-amplify/cli npm path in README

[wuori]

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Needs @ prefix in order to install properly, according to aws-amplify/amplify-cli.

Issue #, if available:

Description of changes:

When trying to install without the @ prefix, NPM fails. Could be a versioning issue on my machine, but the official docs for the amplify-cli uses the @ prefix. This update makes both npm paths match.

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