get-deps.sh: Remove the trailing /download on the zlib URL.

* get-deps.sh:
  (get_zlib): Avoid getting a filename named download.

Suggested by: arfrever


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552958 13f79535-47bb-0310-9956-ffa450edef68