math/big: add function example

While reading the source code of the math/big package, I found the SetString function example of float type missing.

Change-Id: Id8c16a58e2e24f9463e8ff38adbc98f8c418ab26
Reviewed-on: https://go-review.googlesource.com/c/go/+/232804
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/example_test.go

@@ -25,6 +25,13 @@ func ExampleInt_SetString() {
 	// Output: 420
 }
 
+func ExampleFloat_SetString() {
+	f := new(big.Float)
+	f.SetString("3.14159")
+	fmt.Println(f)
+	// Output: 3.14159
+}
+
 func ExampleRat_Scan() {
 	// The Scan function is rarely used directly;
 	// the fmt package recognizes it as an implementation of fmt.Scanner.