Fix typo

Author: brylie

simple_history/models.py

@@ -428,7 +428,7 @@ def get_next_record_diff(self):
             Get the difference between this and the next record. `None` of no next record.
             """
 
-            next_record = self.get_prev_record()
+            next_record = self.get_next_record()
 
             if next_record is not None:
                 return self.diff_against(previous_record)