Fix grammar of get param in set command (#1588)

Author: Andrew-Chen-Wang

redis/commands.py

@@ -1146,7 +1146,7 @@ def set(self, name, value,
             (Available since Redis 6.0)
 
         ``get`` if True, set the value at key ``name`` to ``value`` and return
-            the old value stored at key, or None when key did not exist.
+            the old value stored at key, or None if the key did not exist.
             (Available since Redis 6.2)
 
         ``exat`` sets an expire flag on key ``name`` for ``ex`` seconds,