doc: fix example code

PR-URL: #657
Reviewed-By: Nicola Del Gobbo <[email protected]>
Reviewed-By: Gabriel Schulhof <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>

Author: timarandras

doc/async_worker.md

@@ -409,7 +409,7 @@ The following code shows an example of how to create and use an `Napi::AsyncWork
 // Include EchoWorker class
 // ..
 
-use namespace Napi;
+using namespace Napi;
 
 Value Echo(const CallbackInfo& info) {
     // You need to validate the arguments here.