doc: fix out of date napi_callback doc

The earlier version `napi_callback` returns `void` but now is
`napi_value`. The document of this section hasn't been modified.

Fixes: nodejs#12248

doc/api/n-api.md

@@ -172,7 +172,7 @@ Function pointer type for user-provided native functions which are to be
 exposed to JavaScript via N-API. Callback functions should satisfy the
 following signature:
 ```C
-typedef void (*napi_callback)(napi_env, napi_callback_info);
+typedef napi_value (*napi_callback)(napi_env, napi_callback_info);
 ```
 
 #### *napi_finalize*
@@ -2521,8 +2521,9 @@ In order to expose a function as part of the
 add-on's module exports, set the newly created function on the exports
 object. A sample module might look as follows:
 ```C
-void SayHello(napi_env env, napi_callback_info info) {
+napi_value SayHello(napi_env env, napi_callback_info info) {
   printf("Hello\n");
+  return nullptr;
 }
 
 void Init(napi_env env, napi_value exports, napi_value module, void* priv) {