add comment explaining why .substring(23)

Author: B0pol

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeParsingHelper.java

@@ -436,6 +436,7 @@ public static String getUrlFromNavigationEndpoint(JsonObject navigationEndpoint)
         if (navigationEndpoint.has("urlEndpoint")) {
             String internUrl = navigationEndpoint.getObject("urlEndpoint").getString("url");
             if (internUrl.startsWith("https://www.youtube.com/redirect?")) {
+                // remove https://www.youtube.com part to fall in the next if block
                 internUrl = internUrl.substring(23);
             }