struct incorrect formatting

```
(struct s
  (a     ;; a
   b     ;; b
   c            ;; c
   )
  #:transparent
  #:mutable)
```

is formatted to:

```
(struct s
        (a ;; a
         b ;; b
         c ;; c
         )
  #:transparent
  #:mutable)
```

by @JacobVanGeffen