math: fix typo in Exp2 benchmark

Was timing Exp, not Exp2.

R=rsc, r
CC=golang-dev
https://golang.org/cl/1521042
diff --git a/src/pkg/math/all_test.go b/src/pkg/math/all_test.go
index 15bc68e..9a4801b 100644
--- a/src/pkg/math/all_test.go
+++ b/src/pkg/math/all_test.go
@@ -2339,7 +2339,7 @@
 
 func BenchmarkExp2(b *testing.B) {
 	for i := 0; i < b.N; i++ {
-		Exp(.5)
+		Exp2(.5)
 	}
 }