grpc: minor improvement on WithInsecure() document

[shitian-ni]

Current document asks to replace WithInsecure() with insecure.NewCredentials().
But it causes confusion because function signatures for WithInsecure() and insecure.NewCredentials() are different.

Instead, using insecure.NewCredentials() as a parameter for WithTransportCredentials replicate the deprecated WithInsecure function.

So I propose a minor improvement to mention WithTransportCredentials in the comment.

RELEASE NOTES: None

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Shitian Ni (46ab9df)