docs(gatsby-remark-prismjs): Use Gatsby V2 project structure (#10059)

From the guide "Migrating from v1 to v2" (https://www.gatsbyjs.org/docs/migrating-from-v1-to-v2/#2-move-layoutsindexjs-to-srccomponentslayoutjs-optional-but-recommended). The project structure has the layout file now on "src/components/layout.js" instead of "layouts/index.js"

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packages/gatsby-remark-prismjs/README.md

@@ -137,10 +137,10 @@ CSS along your PrismJS theme and the styles for `.gatsby-highlight-code-line`:
 
 If you want to add line numbering alongside your code, you need to
 import the corresponding CSS file from PrismJS, right after importing your
-colorscheme in `layout/index.js`:
+colorscheme in `src/components/layout.js`:
 
 ```javascript
-// layouts/index.js
+// src/components/layout.js
 require("prismjs/plugins/line-numbers/prism-line-numbers.css")
 ```