system/skeleton: fix /etc/group

When using busybox compiled from current git, login emits the following
message:

    login: /etc/group: bad record

Fix it by adding the missing colon to /etc/group in system/skeleton.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

system/skeleton/etc/group

@@ -21,6 +21,6 @@ plugdev:x:46:
 staff:x:50:
 lock:x:54:
 netdev:x:82:
-ftp:x:83
+ftp:x:83:
 nogroup:x:99:
 users:x:100: