fix broken link

Author: HomelessDinosaur

src/pages/guides/dart/flutter.mdx

@@ -1015,7 +1015,7 @@ region: us-east-1
 
 ### Dockerfile
 
-Because we've mixed Flutter and Dart dependencies, we need to use a [custom container](/docs/reference/custom-containers) that fetches our dependencies using Flutter. You can point to a custom container in your `nitric.yaml`:
+Because we've mixed Flutter and Dart dependencies, we need to use a [custom container](/reference/custom-containers) that fetches our dependencies using Flutter. You can point to a custom container in your `nitric.yaml`:
 
 <Note>
   If you have a separate Dart backend that doesn't share dependencies with your