Change UI test stderr

Author: Rageking8

src/test/ui/parser/issues/issue-101540.stderr

@@ -2,9 +2,8 @@ error: structs are not allowed in struct definitions
   --> $DIR/issue-101540.rs:2:5
    |
 LL |       struct S2 {
-   |  _____-^^^^^
-LL | |
-LL | |     }
-   | |_____- help: consider creating a new `struct` definition instead of nesting
+   |       ^^^^^^^^^
+   |
+   = help: consider creating a new `struct` definition instead of nesting
 
 error: aborting due to previous error