[tz] [PATCH] Don't mention boulder.nist.gov

The generated leapseconds file mentions the data can be retrieved from ftp.boulder.nist.gov/pub/time/leap-seconds.list, but the domain no longer resolves. From the Internet Archive for boulder.nist.gov it seems like it was a redirect for nist.gov until it was later deleted. The attached patch removes reference to it. Collin [1] https://web.archive.org/web/20180901000000*/boulder.nist.gov