using api_url constant in fetch

src/services/ChannelService.ts

@@ -2,7 +2,7 @@ const API_URL = 'http://haveachin.de:1888/v1/channels';
 
 export default class ChannelService {
   public static async fetchAllChannels() {
-    const response = await fetch('http://haveachin.de:1888/v1/channels');
+    const response = await fetch(API_URL);
     const json = await response.json();
     return json;
   }