archive/zip: update the ZIP spec link

Update the link to PKWARE "Application Notes on the .ZIP file format" document.
Now uses the permanent link according to 1.5 in version 6.3.3 (https://pkware.cachefly.net/webdocs/APPNOTE/APPNOTE-6.3.3.TXT):
http://www.pkware.com/appnote

Fixes #18738

Change-Id: If252a5fca1dd666e70c2591a83d8714672d02932
Reviewed-on: https://go-review.googlesource.com/35500Reviewed-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: https://www.pkware.com/documents/casestudies/APPNOTE.TXT
+See: https://www.pkware.com/appnote
 
 This package does not support disk spanning.