Update widgets.py (django-import-export#1193)

Example for widgets usage for export doesn't work.
It has to instantiate class, and correct namespace is widgets, not widget.

Author: renatoch

import_export/widgets.py

@@ -116,7 +116,7 @@ class BooleanWidget(Widget):
         from import_export import fields, resources, widgets
 
         class BooleanExample(resources.ModelResource):
-            warn = fields.Field(widget=widget.BooleanWidget)
+            warn = fields.Field(widget=widgets.BooleanWidget())
 
             def before_row_import(self, row, **kwargs):
                 if "warn" in row.keys():