Change "Change User Password" request example

Changed the new password value in the JSON request for
V3 "Change User Password" example to be more clear about which field
the "new" password should be in and that the user's password will be
that "new" password.

Change-Id: I6790422956ed99f90fd41b6774bd266fd57d7130

api-ref/source/v3/samples/admin/user-password-update-request.json

@@ -1,6 +1,6 @@
 {
     "user": {
-        "password": "old_secretsecret",
+        "password": "new_secretsecret",
         "original_password": "secretsecret"
     }
 }