math/big: fix SetMantExp comment

Change-Id: If30cf9c94b58e18564db46c15c6f5cc14ec1a6fa
Reviewed-on: https://go-review.googlesource.com/16271Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/float.go

@@ -298,7 +298,7 @@ func (z *Float) setExpAndRound(exp int64, sbit uint) {
 // not require 0.5 <= |mant| < 1.0. Specifically:
 //
 //	mant := new(Float)
-//	new(Float).SetMantExp(mant, x.SetMantExp(mant)).Cmp(x).Eql() is true
+//	new(Float).SetMantExp(mant, x.MantExp(mant)).Cmp(x) == 0
 //
 // Special cases are:
 //