docs: fix simple typo, preffix -> prefix (#389)

There is a small typo in googlemaps/directions.py.

Should read `prefix` rather than `preffix`.

Author: timgates42

googlemaps/directions.py

@@ -33,7 +33,7 @@ def directions(client, origin, destination,
 
     :param destination: The address or latitude/longitude value from which
         you wish to calculate directions. You can use a place_id as destination
-        by putting 'place_id:' as a preffix in the passing parameter.
+        by putting 'place_id:' as a prefix in the passing parameter.
     :type destination: string, dict, list, or tuple
 
     :param mode: Specifies the mode of transport to use when calculating