identity: clarify blob format for v3/credentials

The current request/response examples show the credential blob as
a dict for type==ec2, which in incorrect, keystone expects it to
be a json serialized dict, also the description is incorrect, saying
"access_key" when it should be just "access".

These examples align the API docs with what the code actually does,
and also with these docs which say the blob is JSON serialized:
https://github.com/openstack/identity-api/blob/master/\
openstack-identity-api/v3/src/markdown/\
identity-api-v3.md#create-credential-post-credentials

Change-Id: Ieb7b7168f6c041b67a4f43742cb6a540971ff80e
Related-Bug: #1269637