docs: Update Reference to src/layouts/index.js (#19241)

In the the code comments for "showLineNumbers: false", the instruction reference src/layouts/index.js which appears to be left over from Gatsby version 1. The line number instructions further down the page reference gatsby-browser.js instead of src/layouts/index.js. This PR updates the instructions in the code comments to point to gatsby-browser.js.

Author: dcgoodwin2112

packages/gatsby-remark-prismjs/README.md

@@ -40,9 +40,9 @@ plugins: [
             // bash highlighter.
             aliases: {},
             // This toggles the display of line numbers globally alongside the code.
-            // To use it, add the following line in src/layouts/index.js
+            // To use it, add the following line in gatsby-browser.js
             // right after importing the prism color scheme:
-            //  `require("prismjs/plugins/line-numbers/prism-line-numbers.css");`
+            //  require("prismjs/plugins/line-numbers/prism-line-numbers.css")
             // Defaults to false.
             // If you wish to only show line numbers on certain code blocks,
             // leave false and use the {numberLines: true} syntax below