chore(docs): update link to merge-gatsby-config (#24529)

docs/docs/theme-api.md

@@ -135,7 +135,7 @@ Themes at their core are an algorithm that merges multiple `gatsby-config.js` fi
 
 The first example results in a final ordering of `['gatsby-theme-parent', 'gatsby-theme-child']` (parents always come before their children so that children can override functionality), while the second example results in `['gatsby-theme-blog', 'gatsby-theme-notes']`.
 
-Once you have the final ordering of themes you merge them together using a reduce function. [This reduce function](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/utils/merge-gatsby-config.js) specifies the way each key in `gatsby-config.js` will merge together. Unless otherwise specified below, the last value wins.
+Once you have the final ordering of themes you merge them together using a reduce function. [This reduce function](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/utils/merge-gatsby-config.ts) specifies the way each key in `gatsby-config.js` will merge together. Unless otherwise specified below, the last value wins.
 
 - `siteMetadata` and `mapping` both merge deeply using lodash's `merge` function. This means a theme can set default values in `siteMetadata` and the site can override them using the standard `siteMetadata` object in `gatsby-config.js`.
 - `plugins` are normalized to remove duplicates, then concatenated together.