Incorrect direction descriptions for `J` and `K` keybindings

The keybinding descriptions for `J` and `K` in `docs/keybindings/README.md` are reversed.
The document states:

* `J` moves upward
* `K` moves downward

In Vim, the correct behavior is:

* `j` move **down** one line
* `k` move **up** one line

Reference:
[https://github.com/MAIF/yozefu/blob/main/docs/keybindings/README.md](https://github.com/MAIF/yozefu/blob/main/docs/keybindings/README.md)