Fix: some typos in the document (#825)

* Update object_wrap.md

doc/addon.md

@@ -34,7 +34,7 @@ The `Napi::Addon<T>` class can be used together with the `NODE_API_ADDON()` and
 class ExampleAddon : public Napi::Addon<ExampleAddon> {
  public:
   ExampleAddon(Napi::Env env, Napi::Object exports) {
-    // In the constructor we declare the functions the add-on makes avaialable
+    // In the constructor we declare the functions the add-on makes available
     // to JavaScript.
     DefineAddon(exports, {
       InstanceMethod("increment", &ExampleAddon::Increment),

doc/object_wrap.md

@@ -43,7 +43,7 @@ Napi::Object Example::Init(Napi::Env env, Napi::Object exports) {
 
     Napi::FunctionReference* constructor = new Napi::FunctionReference();
 
-    // Create a peristent reference to the class constructor. This will allow
+    // Create a persistent reference to the class constructor. This will allow
     // a function called on a class prototype and a function
     // called on instance of a class to be distinguished from each other.
     *constructor = Napi::Persistent(func);