encoding/base64: fix format error
encoding/base64: fix format error

Found by pending CL to make cmd/vet auto-detect printf wrappers.

Change-Id: I2ad06647b7b41cf68859820a60eeac2e689ca2e6
Reviewed-on: https://go-review.googlesource.com/109344
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,akutz/go,akutz/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,golang/go,josharian/go,akutz/go,nawawi/go,akutz/go,CAFxX/go,golang/go,josharian/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,nawawi/go,golang/go,josharian/go,golang/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,akutz/go,golang/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go