fix a ui test

src/test/ui/structs/incomplete-fn-in-struct-definition.rs

@@ -1,5 +1,5 @@
 fn main() {}
 
 struct S {
-    fn //~ ERROR expected identifier, found keyword `fn`
+    fn: u8 //~ ERROR expected identifier, found keyword `fn`
 }

src/test/ui/structs/incomplete-fn-in-struct-definition.stderr

@@ -1,12 +1,14 @@
 error: expected identifier, found keyword `fn`
   --> $DIR/incomplete-fn-in-struct-definition.rs:4:5
    |
-LL |     fn
+LL | struct S {
+   |        - while parsing this struct
+LL |     fn: u8
    |     ^^ expected identifier, found keyword
    |
 help: escape `fn` to use it as an identifier
    |
-LL |     r#fn
+LL |     r#fn: u8
    |     ++
 
 error: aborting due to previous error