Fix self:: pattern in first function argument

[dtolnay]

error: expected one of: `for`, parentheses, `fn`, `unsafe`, `extern`, identifier, `::`, `<`, `dyn`, square brackets, `*`, `&`, `!`, `impl`, `_`, lifetime
 --> dev/main.rs:7:15
  |
7 |     fn f(self::S: S) {}
  |               ^