Fix spelling mistake

jsfileupload/upload_widget.py

@@ -25,7 +25,7 @@ class FileUpload(DescriptionWidget, ValueWidget):
 
     The widget is able to upload large files by uploading them in chunks.
     The file contents will however not be directly available over the widget,
-    but will have to be read into the notebook seperately.
+    but will have to be read into the notebook separately.
     """
     _model_name = Unicode('FileUploadModel').tag(sync=True)
     _model_module = Unicode(module_name).tag(sync=True)