Fix handling of 3-component OS version numbers.

Get rid of doubled square brackets (what were those for?), and match
dots in the version number with "\.", so we only match dots.

This fixes the handling of version numbers such as 10.10.1.

(A 2-component version number doesn't match, but that's OK - the sed
command doesn't use -n, so that just means it's unchanged by the
command.)

Change-Id: Ia4af97f2f323b88569a1dda6fe9e008c2e408642
Reviewed-on: https://code.wireshark.org/review/5917
Reviewed-by: Guy Harris <guy@alum.mit.edu>