doc: fix order of AtExit callbacks in addons.md

The sanity_check AtExit callback needs to come last to verify that the
other callbacks have been completed. This was not noticed before as this
code was not been executed.

PR-URL: nodejs#14048
Reviewed-By: Michaël Zasso <[email protected]>

Author: danbev

doc/api/addons.md

@@ -1109,10 +1109,10 @@ static void sanity_check(void*) {
 }
 
 void init(Local<Object> exports) {
-  AtExit(sanity_check);
   AtExit(at_exit_cb2, cookie);
   AtExit(at_exit_cb2, cookie);
   AtExit(at_exit_cb1, exports->GetIsolate());
+  AtExit(sanity_check);
 }
 
 NODE_MODULE(addon, init)