directory to run npm install --save XXX from in tutorials

[hazelnusse]

In https://www.gatsbyjs.org/tutorial/part-two/ and https://www.gatsbyjs.org/tutorial/part-three/, it seems that the npm install --save gatsby-plugin-typography and npm install --save gatsby-plugin-typography typography-theme-fairy-gates should be run from within the directory created by the gatsby new command, right? If so, the documentation should probably state this explicitly by adding a cd tutorial-part-two and cd tutorial-part-three, respectively.

[m-allanson]

Thanks for taking this on!

Refs #4577

[KyleAMathews]

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