fix error message for #[custom_mir]

Author: scrabsha

compiler/rustc_attr_parsing/src/attributes/prototype.rs

@@ -82,7 +82,7 @@ fn extract_value<S: Stage>(
     }
 
     let Some(val) = arg.name_value() else {
-        cx.adcx().expected_single_argument(arg.span().unwrap_or(span), 2);
+        cx.adcx().expected_name_value(span, Some(key));
         *failed = true;
         return;
     };