README: stop using ETCDCTL_API=3

Note: v3 has been the default since 25bc65794.

README.md

@@ -105,8 +105,8 @@ This will bring up etcd listening on port 2379 for client communication and on p
 Next, let's set a single key, and then retrieve it:
 
 ```
-ETCDCTL_API=3 etcdctl put mykey "this is awesome"
-ETCDCTL_API=3 etcdctl get mykey
+etcdctl put mykey "this is awesome"
+etcdctl get mykey
 ```
 
 etcd is now running and serving client requests. For more, please check out: