Messages in documentation need updating

According to https://github.com/coreos/etcd/blob/master/Documentation/api.md#atomic-compare-and-swap, the following error message should be "Test Failed":

```
% curl -L http://127.0.0.1:4001/v2/keys/foo\?prevValue\=two -XPUT -d value=three
{"errorCode":101,"message":"Compare failed","cause":"[two != one]","index":67}
```