broken link: fix

typo in the link

_extended-syntax/overview.md

@@ -3,6 +3,6 @@ title: Overview
 syntax-id: overview
 ---
 
-The [basic syntax](/basic-syntax/) outlined in the original Markdown design document added many of the elements needed on a day-to-day basis, but it wasn't enough for some people. That's where extended syntax comes in.
+The [basic syntax](/_basic-syntax/) outlined in the original Markdown design document added many of the elements needed on a day-to-day basis, but it wasn't enough for some people. That's where extended syntax comes in.
 
 Several individuals and organizations took it upon themselves to extend the basic syntax by adding additional elements like tables, code blocks, syntax highlighting, URL auto-linking, and footnotes. These elements can be enabled by using a lightweight markup language that builds upon the basic Markdown syntax, or by adding an extension to a compatible Markdown processor.