needs support for macOS

Log message:

```
bash downloader.sh 
Enter Manga name:
one piece
downloader.sh: line 3: ${manga,,}: bad substitution
Enter the chapter range.
Start:
1
End:
1
Downloading page 1 of chapter 1.....
downloader.sh: line 46: $((length-1)): substring expression < 0
```

Check the expression cited above and fix it.