doc: fix bad lib/time link in 1.12 release notes

There's a "lib/time" sub-section in the Go 1.12 relase notes that
points to a non-existent golang.org/pkg/lib/time page.

The note is about a change in the tz database in the src/lib/time
directory, but the section's title (and the link) should probably just
refer to the time package.

Change-Id: Ibf9dacd710e72886f14ad0b7415fea1e8d25b83a
Reviewed-on: https://go-review.googlesource.com/c/164977


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>