Update index.md (gatsbyjs#10306)

Update imports statements to reflect `emotion/core` instead of `react-emotion` and the commensurate use of `css` property rather than `className`. Also added an explanatory note on the `graphql` sort statement that there are three underscores, instead of two, which the eyes might only see.

<!--
  Q. Which branch should I use for my pull request?
  A. Your best bet is to go for `master`. If you are unsure, ask in the PR, and a Gatsby mantainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->

docs/tutorial/part-six/index.md

@@ -107,7 +107,7 @@ the following to add a GraphQL query with some initial HTML and styling.
 ```jsx:title=src/pages/index.js
 import React from "react"
 import { graphql } from "gatsby"
-import { css } from "react-emotion"
+import { css } from "@emotion/core"
 import { rhythm } from "../utils/typography"
 import Layout from "../components/layout"
 
@@ -117,7 +117,7 @@ export default ({ data }) => {
  <Layout>
  <div>
  <h1
- className={css`
+ css={css`
  display: inline-block;
  border-bottom: 1px solid;
  `}
@@ -128,13 +128,13 @@ export default ({ data }) => {
  {data.allMarkdownRemark.edges.map(({ node }) => (
  <div key={node.id}>
  <h3
- className={css`
+ css={css`
  margin-bottom: ${rhythm(1 / 4)};
  `}
  >
  {node.frontmatter.title}{" "}
  <span
- className={css`
+ css={css`
  color: #bbb;
  `}
  >
@@ -198,7 +198,7 @@ this powerful set of operators, you can select any data you want—in the format
 need.
 
 In your index page's GraphQL query, change `allMarkdownRemark` to
-`allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC })`. Save
+`allMarkdownRemark(sort: { fields: [frontmatter___date], order: DESC })`. _Note: There are 3 underscores between `frontmatter` and `date`._ Save
 this and the sort order should be fixed.
 
 Try opening Graph_i_QL and playing with different sort options. You can sort the