math/big: better SetFloat64 example in doc

Fixes #17221

Change-Id: Idaa2af6b8646651ea72195671d1a4b5c370a5a22
Reviewed-on: https://go-review.googlesource.com/29711
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/doc.go

@@ -31,7 +31,7 @@ setters, for instance:
 
 	var z1 Int
 	z1.SetUint64(123)                 // z1 := 123
-	z2 := new(Rat).SetFloat64(1.2)    // z2 := 6/5
+	z2 := new(Rat).SetFloat64(1.25)   // z2 := 5/4
 	z3 := new(Float).SetInt(z1)       // z3 := 123.0
 
 Setters, numeric operations and predicates are represented as methods of