update docs/api-guide/serializers.md: consistency (encode#6320)

Author: qwhex

docs/api-guide/serializers.md

@@ -208,7 +208,7 @@ To do any other validation that requires access to multiple fields, add a method
 
         def validate(self, data):
             """
-            Check that the start is before the stop.
+            Check that start is before finish.
             """
             if data['start'] > data['finish']:
                 raise serializers.ValidationError("finish must occur after start")