docs(tutorial): fix Highlight-line on wrong line for tutorial part seven (#10635)

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## Description

The hightlight-line comment is on the wrong line at line 77 and should be on line 76. Therefore, it is not showing the highlight of the new getNode function introduced as a parameter to the onCreateNode function.

Screenshot from tutorial on part seven:
<img width="753" alt="screen shot 2018-12-23 at 12 49 14 pm" src="https://user-images.githubusercontent.com/9094342/50386304-1226f880-06b2-11e9-98f2-444bdc859db7.png">

## Related Issues

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  Link to the issue that is fixed by this PR (if there is one)
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Author: wduntak

docs/tutorial/part-seven/index.md

@@ -73,8 +73,8 @@ the "node graph" to its _parent_ `File` node, as `File` nodes contain data you
 need about files on disk. To do that, modify your function again:
 
 ```javascript:title=gatsby-node.js
+// highlight-next-line
 exports.onCreateNode = ({ node, getNode }) => {
-  // highlight-line
   if (node.internal.type === `MarkdownRemark`) {
     // highlight-start
     const fileNode = getNode(node.parent)