math: use ** for exponentiation in comments

R=rsc
CC=golang-dev
https://golang.org/cl/908041
diff --git a/src/pkg/math/ldexp.go b/src/pkg/math/ldexp.go
index dc6b3a8..ab8392a 100644
--- a/src/pkg/math/ldexp.go
+++ b/src/pkg/math/ldexp.go
@@ -5,7 +5,7 @@
 package math
 
 // Ldexp is the inverse of Frexp.
-// It returns frac × 2^exp.
+// It returns frac × 2**exp.
 func Ldexp(frac float64, exp int) float64 {
 	// TODO(rsc): Remove manual inlining of IsNaN, IsInf
 	// when compiler does it for us