lib/xiki/keys.rb: correct location of keybindings

The key bindings are located in @$xiki/lib/xiki/key_bindings.rb, not
@$xiki/key_bindings.rb.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Merge pull request #47 from artagnon/master

Fix location of keybindings