rand: fix printf(var) mistake detected by latest printf checker

For golang/go#69267.

Change-Id: I28b189236bfa7a7efc4a11a8585d358fbce75ec1
Reviewed-on: https://go-review.googlesource.com/c/exp/+/610935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

rand/rand_test.go

@@ -78,7 +78,7 @@ func checkSampleDistribution(t *testing.T, samples []float64, expected *statsRes
 	actual := getStatsResults(samples)
 	err := actual.checkSimilarDistribution(expected)
 	if err != nil {
-		t.Errorf(err.Error())
+		t.Error(err)
 	}
 }