zip: fix typos in documentation

Change-Id: I59d3fcbea6bc0f2d86ccc245c0e23889c03b07e4
Reviewed-on: https://go-review.googlesource.com/c/mod/+/205037
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
1 file changed