update_zlib: Fix regex for detecting versions.

The download link is now a relative link and not absolute. A future
change will get rid of this script and switch to upstream mirroring
from github. We can then use "git merge" to pull in a new version.

Test: Ran manually, checked that 1.2.10 was fetched + unpacked.

Change-Id: I408fa4f9ee7b0dd4c39c635925dc8687950ff0d4