tidy using-gatsby-source-graphql example

[rdela]

Minor cleanup:

• repair links in readme and index pointing to plugins/ instead of packages/
• add fitting name and description in package.json in style of using-gatsby-image:

  gatsby/examples/using-gatsby-image/package.json

  Lines 2 to 4 in 44db2f7

  	"name": "using-gatsby-image",
  	"private": true,
  	"description": "Gatsby example site using using-gatsby-image",

• babel-eslint to ^8.2.2 to remedy various compile errors in yarn and this more informative npm error:

  npm WARN eslint-config-react-app@3.0.0-next.66cc7a90 requires a peer of babel-eslint@^8.2.2 but none is installed. You must install peer dependencies yourself.

  ...seems like we might could go higher: https://github.com/babel/babel-eslint/releases
  ...but this fixes the issues and allows me to build the example in yarn 1.12.3 and npm 6.4.1 on node 10.13.0.

[pieh]

Thanks @rdela!

[gatsbot]

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