Change key name "testAndSet" -> "foo" for clarity

Using `testAndSet` for a key name, as [used in the README](https://github.com/coreos/etcd#atomic-test-and-set) is slightly confusing, in that it may give the impression that the name itself is significant.

I suggest replacing it with something that is obviously an arbitrary name, such as a metasyntactic variable like `foo` (as used elsewhere in the README), or `mykey`. (This pull request changes it to `foo`.)