replaced let with const

lib/single-file/parse-srcset.js

@@ -33,8 +33,8 @@ this.parseSrcset = (() => {
 		}
 
 		function collectCharacters(regEx) {
-			let chars,
-				match = regEx.exec(input.substring(pos));
+			let chars;
+			const match = regEx.exec(input.substring(pos));
 			if (match) {
 				chars = match[0];
 				pos += chars.length;