Update a deprecated link in test file (#1379)

Currently, the URL of converter between **base64** and **HEX** is out of date.
This commit intends to update this URL to the working one.

Co-Authored-By: Nguyen Phuong An <AnNP@vn.fujitsu.com>
Signed-off-by: Kim Bao Long <longkb@vn.fujitsu.com>

model/spanref_test.go

@@ -27,7 +27,7 @@ import (
 func TestSpanRefTypeToFromJSON(t *testing.T) {
 	// base64(0x42, 16 bytes) == AAAAAAAAAAAAAAAAAAAAQg==
 	// base64(0x43, 8 bytes) == AAAAAAAAAEM=
-	// Verify: https://cryptii.com/base64-to-hex
+	// Verify: https://cryptii.com/pipes/base64-to-hex
 	sr := model.SpanRef{
 		TraceID: model.NewTraceID(0, 0x42),
 		SpanID:  model.NewSpanID(0x43),