Fix invalid console.error link (#23213)

packages/next/lib/load-custom-routes.ts

@@ -190,7 +190,7 @@ function checkCustomRoutes(
       console.error(
         `The route ${
           (route as Rewrite).source
-        } rewrites urls outside of the basePath. Please use a destination that starts with \`http://\` or \`https://\` https://err.sh/vercel/next.js/invalid-external-rewrite.md`
+        } rewrites urls outside of the basePath. Please use a destination that starts with \`http://\` or \`https://\` https://err.sh/vercel/next.js/invalid-external-rewrite`
       )
       numInvalidRoutes++
       continue

test/integration/invalid-custom-routes/test/index.test.js

@@ -213,7 +213,7 @@ const runTests = () => {
     )
 
     expect(stderr).toContain(
-      `The route /hello rewrites urls outside of the basePath. Please use a destination that starts with \`http://\` or \`https://\` https://err.sh/vercel/next.js/invalid-external-rewrite.md`
+      `The route /hello rewrites urls outside of the basePath. Please use a destination that starts with \`http://\` or \`https://\` https://err.sh/vercel/next.js/invalid-external-rewrite`
     )
 
     expect(stderr).toContain('Invalid rewrites found')