math: add a Sqrt example

Change-Id: I259e25b9d0b069912053a250e9739e04fafca54d
Reviewed-on: https://go-review.googlesource.com/48892
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/example_test.go

+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math_test
+
+import (
+	"fmt"
+	"math"
+)
+
+func ExampleSqrt() {
+	const (
+		a = 3
+		b = 4
+	)
+	c := math.Sqrt(a*a + b*b)
+	fmt.Printf("%.1f", c)
+	// Output: 5.0
+}