Fix printing of trailing comma after reordered generic params

[dtolnay]

Example:

fn main() {
    let input = "fn f() {}";
    let mut syntax_tree = syn::parse_file(input).unwrap();
    if let syn::Item::Fn(item) = &mut syntax_tree.items[0] {
        item.sig.generics.params.push(syn::parse_quote!(T));
        item.sig.generics.params.push(syn::parse_quote!('a));
    }
    print!("{}", prettyplease::unparse(&syntax_tree));
}

Before:

fn f<'aT, >() {}

After:

fn f<'a, T>() {}