x/crypto/chacha20: fix typo in benchmark
x/crypto/chacha20: fix typo in benchmark

Change-Id: I003c389585a42089fc9cc595d1f5f1027cd482f2
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/272306
Trust: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Run-TryBot: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

golang/crypto,golang/crypto,golang/crypto