math/big: fix comment

The variable is named b not bit.

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/7699043

src/pkg/math/big/int.go

@@ -795,8 +795,8 @@ func (x *Int) Bit(i int) uint {
 }
 
 // SetBit sets z to x, with x's i'th bit set to b (0 or 1).
-// That is, if bit is 1 SetBit sets z = x | (1 << i);
-// if bit is 0 it sets z = x &^ (1 << i). If bit is not 0 or 1,
+// That is, if b is 1 SetBit sets z = x | (1 << i);
+// if b is 0 SetBit sets z = x &^ (1 << i). If b is not 0 or 1,
 // SetBit will panic.
 func (z *Int) SetBit(x *Int, i int, b uint) *Int {
 	if i < 0 {