fix quoted percent sign

src/common.js

@@ -92,7 +92,7 @@ function setup(env) {
 			args[0] = args[0].replace(/%([a-zA-Z%])/g, (match, format) => {
 				// If we encounter an escaped % then don't increase the array index
 				if (match === '%%') {
-					return match;
+					return '%';
 				}
 				index++;
 				const formatter = createDebug.formatters[format];