[1.8.x] Fixed #21588 -- Corrected handler initialization in "modifying upload handlers" example.

Backport of 8f50ff5b15 from master

docs/topics/http/file-uploads.txt

@@ -184,7 +184,7 @@ For instance, suppose you've written a ``ProgressBarUploadHandler`` that
 provides feedback on upload progress to some sort of AJAX widget. You'd add this
 handler to your upload handlers like this::
 
-    request.upload_handlers.insert(0, ProgressBarUploadHandler())
+    request.upload_handlers.insert(0, ProgressBarUploadHandler(request))
 
 You'd probably want to use ``list.insert()`` in this case (instead of
 ``append()``) because a progress bar handler would need to run *before* any
@@ -193,7 +193,7 @@ other handlers. Remember, the upload handlers are processed in order.
 If you want to replace the upload handlers completely, you can just assign a new
 list::
 
-   request.upload_handlers = [ProgressBarUploadHandler()]
+   request.upload_handlers = [ProgressBarUploadHandler(request)]
 
 .. note::
 
@@ -221,7 +221,7 @@ list::
 
         @csrf_exempt
         def upload_file_view(request):
-            request.upload_handlers.insert(0, ProgressBarUploadHandler())
+            request.upload_handlers.insert(0, ProgressBarUploadHandler(request))
             return _upload_file_view(request)
 
         @csrf_protect