Fix wrong comment with DubboBootstrap.useRegistryAsConfigCenterIfNecessary

[laddcn]

What is the purpose of the change

Fix wrong comment with DubboBootstrap.useRegistryAsConfigCenterIfNecessary

Brief changelog

Original comment mentioned "For compatibility purpose, use registry as the default config center when the registry protocol is zookeeper and there's no config center specified explicitly.".
However, it seems this method works ignore the registry protocol because it use registry as the default config when I use nacos as registry protocol.
I debugged this method and found out this method will use registry as the default config when there's no config center specified explicitly and useAsConfigCenter of registryConfig is null or true. So I modified this comment.

Verifying this change

All test cases work fine.

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