uk.kbd: Fix € mapping

We previously emitted the generic currency sign ¤ for Ctrl+4.
Generally speaking the modifer should be AltGr not Ctrl, and this should
be the Euro symbol €.

Fixes: eb8f9028fb38 ("Add the Euro symbol mapping to the UK keyboard keymap.")
Fixes: a232c1d130d2 ("Copy country-code .iso syscons keymaps for vt(4)")
Fixes: fdd869bd8a89 ("Another rpund of fixes, after checking keymaps for plausibility and with")

Reviewed by: ivy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50702