Update source_util.rs

Co-authored-by: Esteban Kuber <estebank@users.noreply.github.com>

Author: kornelski

compiler/rustc_builtin_macros/src/source_util.rs

@@ -238,7 +238,7 @@ fn load_binary_file(
         Err(io_err) => {
             let mut err = cx.dcx().struct_span_err(
                 macro_span,
-                format!("couldn't read {}: {io_err}", resolved_path.display()),
+                format!("couldn't read `{}`: {io_err}", resolved_path.display()),
             );
 
             if original_path.is_relative() {