typo: update upgrade-guide.mdx (#2211)

Removed a broken link reference.

Author: davidsneighbour

src/docs/upgrade-guide.mdx

@@ -78,7 +78,7 @@ npx @tailwindcss/cli -i input.css -o output.css
 
 Here's a comprehensive list of all the breaking changes in Tailwind CSS v4.0.
 
-Our [upgrade tool](##using-the-upgrade-tool) will handle most of these changes for you automatically, so we highly recommend using it if you can.
+Our [upgrade tool](#using-the-upgrade-tool) will handle most of these changes for you automatically, so we highly recommend using it if you can.
 
 ### Browser requirements