Yicheng Qin 147885078c etcdctl: fix watch -after-index parsing
It uses -after-index incorrectly now:

```
$ ./bin/etcdctl --debug watch -after-index 31 foo
Cluster-Endpoints: http://localhost:2379, http://localhost:4001
cURL Command: curl -X GET
http://localhost:2379/v2/keys/foo?recursive=false&wait=true&waitIndex=33
```

After this PR:

```
$ ./bin/etcdctl --debug watch -after-index 31 foo
Cluster-Endpoints: http://localhost:2379, http://localhost:4001
cURL Command: curl -X GET
http://localhost:2379/v2/keys/foo?recursive=false&wait=true&waitIndex=32
```
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