fix test case issue ref

Author: wecing

src/test/ui/mir/issue-78496.rs

@@ -1,7 +1,7 @@
 // run-pass
 // compile-flags: -Z mir-opt-level=2 -C opt-level=0
 
-// example from #68867
+// example from #78496
 pub enum E<'a> {
     Empty,
     Some(&'a E<'a>),