Update files/en-us/web/api/fetch_api/using_fetch/index.md

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>

files/en-us/web/api/fetch_api/using_fetch/index.md

@@ -481,7 +481,7 @@ async function* makeTextFileLineIterator(fileURL) {
 
   if (startIndex < chunk.length) {
     // Last line didn't end in a newline char
-    yield chunk.substr(startIndex);
+    yield chunk.substring(startIndex);
   }
 }