Outdated URL in the 1.6 Dockerfile comment

[jusle]

Line 9 should be
# see http://sources.debian.net/src/haproxy/1.6.4-3/debian/rules/ for some helpful navigation of the possible "make" arguments instead of # see http://sources.debian.net/src/haproxy/1.5.8-1/debian/rules/ for some helpful navigation of the possible "make" arguments.

[tianon]

This was mostly fixed via #38, and finished via #63. 👍

(Using "https://sources.debian.net/src/haproxy/jessie/debian/rules/" instead, which will auto-resolve to the latest release in the jessie distribution.)