index/suffixarray: add Lookup example

Updates #16360

Change-Id: Idd8523b5a9a496ebd9c6e3b89c30df539842a139
Reviewed-on: https://go-review.googlesource.com/27433Reviewed-by: C Cirello <uldericofilho@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/index/suffixarray/example_test.go

+// Copyright 2016 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 suffixarray_test
+
+import (
+	"fmt"
+	"index/suffixarray"
+)
+
+func ExampleIndex_Lookup() {
+	index := suffixarray.New([]byte("banana"))
+	offsets := index.Lookup([]byte("ana"), -1)
+	for _, off := range offsets {
+		fmt.Println(off)
+	}
+
+	// Unordered output:
+	// 1
+	// 3
+}