Update comment for AdamW implementation reference. (#4604)

Co-authored-by: Will Findley <findley@gmail.com>

ggml.c

@@ -17456,9 +17456,9 @@ static void ggml_opt_acc_grad(int np, struct ggml_tensor * const ps[], float * g
 }
 
 //
-// ADAM
+// Using AdamW - ref: https://arxiv.org/pdf/1711.05101v3.pdf
 //
-//   ref: https://arxiv.org/pdf/1412.6980.pdf
+// (Original Adam - ref: https://arxiv.org/pdf/1412.6980.pdf)
 //
 
 static enum ggml_opt_result ggml_opt_adam(