Altered the footer error message for ui/test/spanissue-39018.stderr as per what compiler produces now.

Author: Ruster-a11y

src/test/ui/span/issue-39018.stderr

@@ -185,5 +185,5 @@ LL |     let _ = c.to_owned() + d;
 
 error: aborting due to 14 previous errors
 
-Some errors have detailed explanations: E0308, E0369.
-For more information about an error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0277, E0369.
+For more information about an error, try `rustc --explain E0277`.