Fix spacing and ordering of words in pretty printed Impl

[dtolnay]

Follow-up to #92238 fixing one of the FIXMEs.

macro_rules! repro {
    ($item:item) => {
        stringify!($item)
    };
}

fn main() {
    println!("{}", repro!(impl<T> Struct<T> {}));
    println!("{}", repro!(impl<T> const Trait for T {}));
}

Before: impl <T> Struct<T> {}
After: impl<T> Struct<T> {}

Before: impl const <T> Trait for T {} 😿
After: impl<T> const Trait for T {}

[rust-highfive]

r? @jackh726

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[jackh726]

@bors r+ rollup

[bors]

📌 Commit a24e238 has been approved by jackh726