Change missing trailing comma span to point to span where comma is missing from

Signed-off-by: Torstein Grindvik <torstein.grindvik@nordicsemi.no>

packages/core-macro/tests/rsx.rs

@@ -0,0 +1,5 @@
+#[test]
+fn rsx() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/rsx/trailing-comma-0.rs");
+}