Fix small error in documentation (vercel#46105)

Just fixing a small comment in the doc

docs/basic-features/data-fetching/incremental-static-regeneration.md

@@ -73,7 +73,7 @@ export async function getStaticPaths() {
   }))
 
   // We'll pre-render only these paths at build time.
-  // { fallback: blocking } will server-render pages
+  // { fallback: 'blocking' } will server-render pages
   // on-demand if the path doesn't exist.
   return { paths, fallback: 'blocking' }
 }