Revert "Use npm instead of yarn in Style Guide"

This reverts commit ac72f11.

For our internal stuff, we do want people to use Yarn as we commit yarn.lock files.

docs/docs/gatsby-style-guide.md

@@ -86,10 +86,10 @@ repository.
 Finally, if you want to run a version of the Guides repository locally, follow
 these steps:
 
-1. Install the Gatsby command-line tool `npm install --global gatsby-cli`.
-2. Follow the steps above to fork and clone the Gatsby repo.
-3. Enter the `www` directory where gatsbyjs.org is by `cd path-to-gatsby/www` and install its dependencies with `npm install`.
-4. Run `gatsby develop` and view Guides locally.
+1. Ensure you have the yarn package manager installed `npm install -g yarn`
+2. Install the Gatsby cli `yarn add --global gatsby-cli`
+3. Follow the steps above to fork and clone the Gatsby repo.
+4. Enter the `www` directory where gatsbyjs.org is and install its dependencies `yarn install` then run `gatsby develop`.
 
 ## Article style guide