fix formatting

MIGRATION.md

@@ -97,7 +97,7 @@
   instead of
 
   ```rust
-fn index(req: &HttpRequest) -> Box<Future<Item=HttpResponse, Error=Error>> {
+  fn index(req: &HttpRequest) -> Box<Future<Item=HttpResponse, Error=Error>> {
     req
        .payload()
        .from_err()
@@ -106,21 +106,21 @@ fn index(req: &HttpRequest) -> Box<Future<Item=HttpResponse, Error=Error>> {
         })
        .map(|_| HttpResponse::Ok().finish())
        .responder()
-}
-   ```
+  }
+  ```
 
-   use `Payload` extractor
+  use `Payload` extractor
 
   ```rust
-fn index(stream: web::Payload) -> impl Future<Item=HttpResponse, Error=Error> {
-    stream
+  fn index(stream: web::Payload) -> impl Future<Item=HttpResponse, Error=Error> {
+     stream
        .from_err()
        .fold((), |_, chunk| {
             ...
         })
        .map(|_| HttpResponse::Ok().finish())
-}
-   ```
+  }
+  ```
 
 * `State` is now `Data`.  You register Data during the App initialization process
   and then access it from handlers either using a Data extractor or using