math: change Nextafter64 to Nextafter in the description of Nextafter

Change-Id: I3419d6247fbff36aa1ed5451bb3cfb7502c3d07e
Reviewed-on: https://go-review.googlesource.com/5030Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/math/nextafter.go

@@ -29,9 +29,9 @@ func Nextafter32(x, y float32) (r float32) {
 // Nextafter returns the next representable float64 value after x towards y.
 //
 // Special cases are:
-//	Nextafter64(x, x)   = x
-//	Nextafter64(NaN, y) = NaN
-//	Nextafter64(x, NaN) = NaN
+//	Nextafter(x, x)   = x
+//	Nextafter(NaN, y) = NaN
+//	Nextafter(x, NaN) = NaN
 func Nextafter(x, y float64) (r float64) {
 	switch {
 	case IsNaN(x) || IsNaN(y): // special case