fix(docs): 404 links to gatsby-themes (#26273)

docs/blog/2019-07-15-theme-jam-contest/index.md

@@ -15,11 +15,11 @@ To celebrate the [stable release of Gatsby Themes](/blog/2019-07-03-announcing-s
 
 Gatsby Themes are pre-built, installable packages for setting up a site’s default styling, components, plugins, and overall configuration that can be reused across multiple Gatsby sites. They’re a great way to simplify the early steps of configuring a new site; and, when it’s time to update your site’s styling or functionality, you only have to edit your theme or swap it out entirely with another compatible theme. You can find a more detailed explanation in the [Themes section of Gatsby’s documentation](/docs/themes/).
 
-If you’d like to see an example, check out Gatsby’s official [blog theme](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-theme-blog) or [notes theme](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-theme-notes).
+If you’d like to see an example, check out Gatsby’s official [blog theme](https://github.com/gatsbyjs/themes/tree/master/packages/gatsby-theme-blog) or [notes theme](https://github.com/gatsbyjs/themes/tree/master/packages/gatsby-theme-notes).
 
 ## How to build your Gatsby theme
 
-If you’re planning to build a theme for the contest, you may want to start with this [Gatsby Theme Jam Submission Example repository](https://github.com/jlengstorf/gatsby-theme-jam-example). You can build a child theme of an existing Gatsby theme (e.g. the [blog theme](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-theme-blog) or [notes theme](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-theme-notes)) with updated styling or added functionality, for example, or it can be a completely new theme.
+If you’re planning to build a theme for the contest, you may want to start with this [Gatsby Theme Jam Submission Example repository](https://github.com/jlengstorf/gatsby-theme-jam-example). You can build a child theme of an existing Gatsby theme (e.g. the [blog theme](https://github.com/gatsbyjs/themes/tree/master/packages/gatsby-theme-blog) or [notes theme](https://github.com/gatsbyjs/themes/tree/master/packages/gatsby-theme-notes)) with updated styling or added functionality, for example, or it can be a completely new theme.
 
 Here are some resources with detailed instructions on how to build a Gatsby theme: