Fix observable sample code (#246)

Fixes #210.

Author: andywer

docs/usage-observables.md

@@ -21,7 +21,7 @@ import { spawn, Thread, Worker } from "threads"
 
 const counter = await spawn(new Worker("./workers/counter"))
 
-counter.subscribe(newCount => console.log(`Counter incremented to:`, newCount))
+counter().subscribe(newCount => console.log(`Counter incremented to:`, newCount))
 ```
 
 ```js