doc: use https scheme in oss-distros link

The https scheme is supported, and should be used per best practices.

The previous http link redirected to https:

	$ curl -i 'http://oss-security.openwall.org/wiki/mailing-lists/distros'
	HTTP/1.1 302 Moved Temporarily
	Location: https://oss-security.openwall.org/wiki/mailing-lists/distros

Change-Id: I857b93eeec45996d6dc05dbf7532d1759bf4d447
Reviewed-on: https://go-review.googlesource.com/c/153457


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>