navigation vim: lock fn layer

[bouttier]

The documentation said the Fn layer should be locked :

arsenik/README.md

Lines 147 to 148 in d433298

	<kbd>NumPad</kbd> and <kbd>Fn</kbd> lock these layers: they remain active
	without holding the key until escaped with <kbd>Alt</kbd> or <kbd>AltGr</kbd>.

This PR fix the config to match the doc.

But I prefer than this PR #50!

[fnuttens]

Good catch, however I believe the funpad works better as a layer-while-held. Most of the time, I just want to hit a single Fn key; having to manually leave the layer would feel a bit cumbersome. It’s not like the numpad where you’re often staying for more than 2 keys. Maybe we should change the README instead?

Edit: I just went through #50 and like this proposal much better 🙂