math/big: Correcting typo in SetBit() documentation
math/big: Correcting typo in SetBit() documentation

z is set to x+bit change, not z+bit change

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/5374056

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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