doc: fix example code

PR-URL: nodejs/node-addon-api#657
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

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.