Minor typo

rest_framework/fields.py

@@ -289,7 +289,7 @@ class WritableField(Field):
         self.validators = self.default_validators + validators
         self.default = default if default is not None else self.default
 
-        # Widgets are ony used for HTML forms.
+        # Widgets are only used for HTML forms.
         widget = widget or self.widget
         if isinstance(widget, type):
             widget = widget()