In the sample code demonstrating how to specify a client request
timeout, the `cancel()` is called immediately after the Put, but it
should be deferred instead, giving the Put enough time to complete.
In the canonical Golang context
[docs](https://pkg.go.dev/context#WithTimeout), the sample code sets a
`defer cancel()` immediately after context creation, and with this
commit we adhere to that convention.
fixes:
```json
{
"level": "warn",
"ts": "2021-12-29T09:56:42.439-0800",
"logger": "etcd-client",
"caller": "v3@v3.5.1/retry_interceptor.go:62",
"msg": "retrying of unary invoker failed",
"target": "etcd-endpoints://0xc000213340/localhost:2379",
"attempt": 0,
"error": "rpc error: code = Canceled desc = context canceled"
}
```
1) Fix the path for rpc errors
2) The README says 2 types of client errors vs doc mentioning 3
types of client errors. grpc erros should be considered single time
as covered in the README.
Benjamin Wang