Assign prev_results to correct member in TrasnportOperator's constructor

Author: kingyue737

openmc/deplete/abc.py

@@ -122,7 +122,7 @@ def __init__(self, chain_file=None, fission_q=None, dilute_initial=1.0e3,
             self.prev_res = None
         else:
             check_type("previous results", prev_results, ResultsList)
-            self.prev_results = prev_results
+            self.prev_res = prev_results
 
     @property
     def dilute_initial(self):