describe the go get step before cue get go in the tutorial

[docteurklein]

see #95. Still needs to be done for the docs in cuelang.org website.

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[docteurklein]

@googlebot I signed it!

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