x/crypto/chacha20: fix typo in benchmark

Change-Id: I003c389585a42089fc9cc595d1f5f1027cd482f2
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/272306
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
diff --git a/chacha20/chacha_test.go b/chacha20/chacha_test.go
index d75873c..60b11d9 100644
--- a/chacha20/chacha_test.go
+++ b/chacha20/chacha_test.go
@@ -238,7 +238,7 @@
 		benchmarkChaCha20(b, 10, 25)
 	})
 	b.Run("4096", func(b *testing.B) {
-		benchmarkChaCha20(b, 256, 1)
+		benchmarkChaCha20(b, 4096, 1)
 	})
 	b.Run("100x40", func(b *testing.B) {
 		benchmarkChaCha20(b, 100, 40)