[css-values-5] Refer to attr-type argument, not syntax (#11833)

There is no `<syntax>` argument to attr() anymore;
refer to `<attr-type>` instead.

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>

Author: andruud

css-values-5/Overview.bs

@@ -1646,7 +1646,7 @@ Ian's proposal:
 		if the attribute is missing
 		or fails to parse as the specified type.
 
-		If the <<syntax>> argument is omitted,
+		If the <<attr-type>> argument is omitted,
 		the fallback defaults to the empty string if omitted;
 		otherwise, it defaults to the [=guaranteed-invalid value=] if omitted.