Fix example in serializers.md

docs/api-guide/serializers.md

@@ -209,13 +209,13 @@ To do any other validation that requires access to multiple fields, add a method
         start = serializers.DateTimeField()
         finish = serializers.DateTimeField()
 
-        def validate(self, data):
+        def validate(self, attrs):
             """
             Check that start is before finish.
             """
-            if data['start'] > data['finish']:
+            if attrs['start'] > attrs['finish']:
                 raise serializers.ValidationError("finish must occur after start")
-            return data
+            return attrs
 
 #### Validators