Fix example uploadr - wrong FieldList import

[staeff]

The uploadr example failed with the following error message:

~/workspace/examples/uploadr$ python app.py   
Traceback (most recent call last):
      File "app.py", line 2, in <module>        
         from flask_wtf import Form, FieldList
ImportError: cannot import name FieldList

Changing the import for FieldList removed the error.