Fix broken link for Markdown Guide

doc/development/documentation/styleguide/index.md

@@ -112,7 +112,7 @@ features is limited by our linters, so, use regular Markdown and follow the rule
 linked style guide. You can't use Kramdown-specific markup (for example, `{:.class}`).
 
 For a complete Kramdown reference, see the
-[GitLab Markdown Guide](https://about.gitlab.com/handbook/markdown-guide/).
+[GitLab Markdown Guide](https://handbook.gitlab.com/handbook/markdown-guide/).
 
 The Markdown format is [tested](../testing.md) by using markdownlint and Vale.