style-guide: fix typo for empty struct advice

the advice appears to apply to empty structs with braces (parens/blocks), and
a unit struct in the comment does not make sense. Fix the typo.

Author: fee1-dead

src/doc/style-guide/src/items.md

@@ -123,7 +123,7 @@ struct Foo {
 Prefer using a unit struct (e.g., `struct Foo;`) to an empty struct (e.g.,
 `struct Foo();` or `struct Foo {}`, these only exist to simplify code
 generation), but if you must use an empty struct, keep it on one line with no
-space between the braces: `struct Foo;` or `struct Foo {}`.
+space between the braces: `struct Foo();` or `struct Foo {}`.
 
 The same guidelines are used for untagged union declarations.