update the docs as well

Author: ofrobots

doc/maybe_types.md

@@ -130,7 +130,7 @@ Signature:
 
 ```c++
 Nan::MaybeLocal<v8::Value> Nan::Call(v8::Local<v8::Function> fun, v8::Local<v8::Object> recv, int argc, v8::Local<v8::Value> argv[]);
-Nan::MaybeLocal<v8::Value> Nan::Call(Nan::Callback* callback, v8::Local<v8::Object> recv,
+Nan::MaybeLocal<v8::Value> Nan::Call(const Nan::Callback& callback, v8::Local<v8::Object> recv,
  int argc, v8::Local<v8::Value> argv[]);
 ```