fix: correct KMS keyring parameter name from child_key_ids to key_ids

[mattsb42-aws]

Issue #, if available:
resolves: #220

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[WesleyRosenblum]

The spec just refers to this as "Key Ids" which is what I called it in Java. What is it called in C and Javascript?

[mattsb42-aws]

C calls it additional_key_ids

https://github.com/aws/aws-encryption-sdk-c/blob/749c84feddce147fb62e2ece7de34d733ef165c2/aws-encryption-sdk-cpp/include/aws/cryptosdk/cpp/kms_keyring.h#L129

JS calls it keyIds

https://github.com/aws/aws-encryption-sdk-javascript/blob/201c1022257a08c82035e5cde5097fad8321f64d/modules/kms-keyring-node/src/kms_keyring_node.ts#L42

https://github.com/aws/aws-encryption-sdk-javascript/blob/201c1022257a08c82035e5cde5097fad8321f64d/modules/kms-keyring-browser/src/kms_keyring_browser.ts#L49

I'm conflicted on the naming for this. In the context of also involving a generator key ID, I think that "additional" makes more sense because "key IDs" is just too undifferentiated. But, without the context of a generator ID, "key IDs" makes more sense because they are not in addition to anything.