Lint/fix typo

Author: Brylie Christopher Oxley

simple_history/models.py

@@ -425,13 +425,15 @@ def get_next_record(self):
 
         def get_next_record_diff(self):
             """
-            Get the difference between this and the next record. `None` of no next record.
+            Get the difference between this and the next record. 
+            
+            Return the diff or `None` of no next record.
             """
 
             next_record = self.get_next_record()
 
             if next_record is not None:
-                return self.diff_against(previous_record)
+                return self.diff_against(next_record)
 
             return None
 
@@ -448,9 +450,12 @@ def get_prev_record(self):
 
         def get_prev_record_diff(self):
             """
-            Get the difference between this and the previous record. `None` if no previous record.
+            Get the difference between this and the previous record. 
+            
+            The diff should be based on the previous record, 
+            since the diff uses `old_record` and `new_record` semantics.
 
-            The diff should be based on the previous record, since the diff uses `old_record` and `new_record` semantics.
+            Return the diff or `None` if no previous record.
             """
 
             previous_record = self.get_prev_record()