sw-precache: `git clone` is missing between step 1 & 2

[nathanbrauer]

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Codelab Affected: Adding a Service Worker with sw-precache

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Step 1 does not contain any instruction asking you to clone any repo:

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Step 2 jumps straight to "With the code downloaded ... Checkout the code-lab branch...":

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Potential solution

I'm guessing this is the repo that needs to be cloned first:

git clone https://github.com/GoogleChrome/airhorn.git

cc @brandon32 @PaulKinlan @sarahec