doc: change node.js to Node.js per guideline (#579)

PR-URL: nodejs/node-addon-api#579

Reviewed-By: NickNaso <nicoladelgobbo@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

doc/async_operations.md

@@ -4,7 +4,7 @@ Node.js native add-ons often need to execute long running tasks and to avoid
 blocking the **event loop** they have to run them asynchronously from the
 **event loop**.
 In the Node.js model of execution the event loop thread represents the thread
-where JavaScript code is executing. The node.js guidance is to avoid blocking
+where JavaScript code is executing. The Node.js guidance is to avoid blocking
 other work queued on the event loop thread. Therefore, we need to do this work on
 another thread.