Fix mismatched punctuation in Debug impl of AttrId

Author: dtolnay

compiler/rustc_ast/src/ast.rs

@@ -2570,7 +2570,7 @@ pub enum AttrStyle {
 
 rustc_index::newtype_index! {
     #[custom_encodable]
-    #[debug_format = "AttrId({})]"]
+    #[debug_format = "AttrId({})"]
     pub struct AttrId {}
 }