Declare variable key in the get_user example (donnemartin#26)

Author: RoxasShadow

README.md

@@ -1156,6 +1156,7 @@ def get_user(self, user_id):
     if user is None:
         user = db.query("SELECT * FROM users WHERE user_id = {0}", user_id)
         if user is not None:
+            key = "user.{0}".format(user_id)
             cache.set(key, json.dumps(user))
     return user
 ```