Signal example should use syscall.SIGTERM instead of os.Kill

Regarding the documented signals example, os.Kill can't be caught, it's just there for killing other processes.

os.Kill is like `kill -9` which kills a process immediately

syscall.SIGTERM is equivalent to `kill` which allows the process time to cleanup

```go
signal.Notify(sigchan, os.Interrupt, syscall.SIGTERM)
```

ref: #9 
