update example-preseed.txt to mention partman-lvm/confirm_nooverwrite

[Mathieu Parent <math.parent@gmail.com>]

Hi,

File http://www.debian.org/releases/stable/example-preseed.txt should
be also updated by adding the following line:
d-i partman-lvm/confirm_nooverwrite boolean true

after:
d-i partman-lvm/confirm boolean true

But I don't knnow where to report this.

Regards
-- 
Mathieu Parent