Fix inconsistent quotes in Vue config

frontend/vue.config.js

@@ -18,8 +18,8 @@ module.exports = {
 	devServer: {
     port: process.env.LISTMONK_FRONTEND_PORT || 8080,
 		proxy: {
-			"^/api": {
-				target: process.env.LISTMONK_API_URL || "http://127.0.0.1:9000"
+			'^/api': {
+				target: process.env.LISTMONK_API_URL || 'http://127.0.0.1:9000'
 			}
 		}
 	}