fix: add leading slash to react-developer-tools link in setup guide

The link was missing the leading slash, making it a relative path
that wouldn't resolve correctly. Changed from 'learn/...' to '/learn/...'
for consistency with other links in the document.

Author: REAPER2705

src/content/learn/setup.md

@@ -17,7 +17,7 @@ TypeScript is a popular way to add type definitions to JavaScript codebases. [Le
 
 ## React Developer Tools {/*react-developer-tools*/}
 
-React Developer Tools is a browser extension that can inspect React components, edit props and state, and identify performance problems. Learn how to install it [here](learn/react-developer-tools).
+React Developer Tools is a browser extension that can inspect React components, edit props and state, and identify performance problems. Learn how to install it [here](/learn/react-developer-tools).
 
 ## React Compiler {/*react-compiler*/}