Use Oxford comma properly in getex (#1586)

Author: Andrew-Chen-Wang

redis/commands.py

@@ -843,7 +843,7 @@ def getex(self, name,
 
         opset = set([ex, px, exat, pxat])
         if len(opset) > 2 or len(opset) > 1 and persist:
-            raise DataError("``ex``, ``px``, ``exat``, ``pxat``",
+            raise DataError("``ex``, ``px``, ``exat``, ``pxat``, "
                             "and ``persist`` are mutually exclusive.")
 
         pieces = []