archive/zip: use HTTPS for documentation link

The resource is available over (and redirects to) HTTPS, it seems like a good
idea to save a redirect and ensure an encrypted connection.

Change-Id: I262c7616ae289cdd756b6f67573ba6bd7e3e0ca6
Reviewed-on: https://go-review.googlesource.com/23104Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/archive/zip/struct.go

@@ -5,7 +5,7 @@
 /*
 Package zip provides support for reading and writing ZIP archives.
 
-See: http://www.pkware.com/documents/casestudies/APPNOTE.TXT
+See: https://www.pkware.com/documents/casestudies/APPNOTE.TXT
 
 This package does not support disk spanning.