PYTHON-2628: Fix 'encryption::create_data_key` docstring to use existing algorithm

(cherry picked from commit 97bad5a)

Author: guanlinzhou

pymongo/encryption.py

@@ -491,7 +491,7 @@ def create_data_key(self, kms_provider, master_key=None,
               client_encryption.create_data_key("local", keyAltNames=["name1"])
               # reference the key with the alternate name
               client_encryption.encrypt("457-55-5462", keyAltName="name1",
-                                        algorithm=Algorithm.Random)
+                                        algorithm=Algorithm.AEAD_AES_256_CBC_HMAC_SHA_512_Random)
 
         :Returns:
           The ``_id`` of the created data key document as a