tools: use template literal in error message

PR-URL: #15846
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Tim Chon

tools/doc/generate.js

@@ -68,6 +68,6 @@ function next(er, input) {
       break;
 
     default:
-      throw new Error('Invalid format: ' + format);
+      throw new Error(`Invalid format: ${format}`);
   }
 }