Keybinding to transpose current line with next line

[Christopher Dimech]

I have written the following keybinding to swap lines. If I have
the cursor on one line, the command swaps the current line with
the previous line.   What can I do for the keybinding to swap
the current line with the next line, rather than with the previous one.

  (global-set-key (kbd "C-t l")   'transpose-lines)