doc: fix typo in binding functions

According to usage(node::util::RegisterExternalReferences) of
below line, namespace has to be `util`(not `utils`).

PR-URL: #48003
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: deokjinkim

src/README.md

@@ -432,7 +432,7 @@ with the utilities in `node_external_reference.h`, like this:
 
 ```cpp
 namespace node {
-namespace utils {
+namespace util {
 void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
   registry->Register(GetHiddenValue);
   registry->Register(SetHiddenValue);