doc: update tzdata version in 1.12 release notes

It was recently updated (again) to version 2018i. Since we're here,
wrap the paragraph at ~70 columns, like all the others.

Change-Id: I0a380385f34f1df1258a9f2af447234967422f37
Reviewed-on: https://go-review.googlesource.com/c/156857


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