math: fix typo in all_test.go
math: fix typo in all_test.go

Logb errors were reported as Ilogb errors.

R=rsc, golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5517045

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go