tools: use template literal in error message

PR-URL: nodejs/node#15846
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

tools/doc/generate.js

@@ -89,6 +89,6 @@ function next(er, input) {
       break;
 
     default:
-      throw new Error('Invalid format: ' + format);
+      throw new Error(`Invalid format: ${format}`);
   }
 }