make udp broadcast work as expected

The broadcast property is not enabled when specifying 
```clojure 
(udp-socket {:broadcast true})
```
There is a tiny mismatch between the documentation and code. 

```clojure
{:keys [port broadcast? buf-size]
         :or {port 0
              buf-size 16384}} options
```

An alternative solution would be to update the doc so it says 'broadcast?' rather than 'broadcast'. 

Ex:
```clojure
(udp-socket {:broadcast? true})
```