Tzinfo was replaced with TZData during the review of CL 68890, but this
instance was forgotten. Update it for consistency.

Follows CL 68890.
Updates #20629.

Change-Id: Id6d3c4f5f7572b01065f2db556db605452d1b570
Reviewed-on: https://go-review.googlesource.com/79176Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>