Update getting-started.md

[codenameEli]

Running through the setup and accidentally ran npm install --save @angular/material in my projects folder, quickly realized i forgot to cd in there. Thought it would be frustrating if someone ran through the entire process of setting up without cd-ing.

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[codenameEli]

I signed it!

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CLAs look good, thanks!

[jelbourn]

#3523 removes the ng new from the getting started guide

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