math: use ** for exponentiation in comments

R=rsc
CC=golang-dev
https://golang.org/cl/908041
diff --git a/src/pkg/math/log.go b/src/pkg/math/log.go
index 1727c77..02e767b 100644
--- a/src/pkg/math/log.go
+++ b/src/pkg/math/log.go
@@ -27,7 +27,7 @@
 //
 // Method :
 //   1. Argument Reduction: find k and f such that
-//			x = 2^k * (1+f),
+//			x = 2**k * (1+f),
 //	   where  sqrt(2)/2 < 1+f < sqrt(2) .
 //
 //   2. Approximation of log(1+f).