doc: fix minor typo in object_wrap.md (#741)

PR-URL: #741
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>

Author: danbev

doc/object_wrap.md

@@ -137,7 +137,7 @@ static T* Napi::ObjectWrap::Unwrap(Napi::Object wrapper);
 
 *`[in] wrapper`: The JavaScript object that wraps the native instance.
 
-Returns a native instace wrapped in a JavaScript object. Given the
+Returns a native instance wrapped in a JavaScript object. Given the
 Napi:Object, this allows a method to get a pointer to the wrapped
 C++ object and then reference fields, call methods, etc. within that class.
 In many cases calling Unwrap is not required, as methods can