regexp: trivial change in comments to update code.google.com link

regexp: trivial change in comments to update code.google.com link

Replaced code.google.com/p/re2/ with github.com/google/re2/ and
updated the file names (re2-exhaustive.txt.bz2 not re2.txt.gz)
as well as the re2 make command (make log).

Change-Id: I15937b0b8a898d78d45366857ed86421c8d69960
Reviewed-on: https://go-review.googlesource.com/9372
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>