Fix variable shadowing in blog-starter example (#13988)

Sorry for the super trivial PR. I noticed this while working with the example and figured it might be worth fixing. 

examples/blog-starter/pages/posts/[slug].js

@@ -72,10 +72,10 @@ export async function getStaticPaths() {
   const posts = getAllPosts(['slug'])
 
   return {
-    paths: posts.map((posts) => {
+    paths: posts.map((post) => {
       return {
         params: {
-          slug: posts.slug,
+          slug: post.slug,
         },
       }
     }),