math/big: fix typo

Found by github user asukakenji.

Change-Id: I4c76316b69e8a243fb6bf280283f3722e728d853
Reviewed-on: https://go-review.googlesource.com/17641Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/float.go

@@ -1123,7 +1123,7 @@ func (x *Float) Int(z *Int) (*Int, Accuracy) {
 
 // Rat returns the rational number corresponding to x;
 // or nil if x is an infinity.
-// The result is Exact is x is not an Inf.
+// The result is Exact if x is not an Inf.
 // If a non-nil *Rat argument z is provided, Rat stores
 // the result in z instead of allocating a new Rat.
 func (x *Float) Rat(z *Rat) (*Rat, Accuracy) {