archive/zip: hyperlink is outdated

[catalase]

https://golang.org/pkg/archive/zip

the link to zip spec on the page is outdated.

[ibrasho]

This is the permanent link: https://www.pkware.com/appnote

[bradfitz]

https://go-review.googlesource.com/c/35500/

[gopherbot]

CL https://golang.org/cl/35500 mentions this issue.