sha3: added example output

Output for sha3.ShakeSum256 and sha3.NewShake256 examples is now
verified.

Change-Id: I31e186a025cbea20d206a253fccea44e1f8a6e49
Reviewed-on: https://go-review.googlesource.com/48870
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

sha3/sha3_test.go

@@ -15,6 +15,7 @@ import (
 	"compress/flate"
 	"encoding/hex"
 	"encoding/json"
+	"fmt"
 	"hash"
 	"os"
 	"strings"
@@ -289,6 +290,8 @@ func Example_sum() {
 	h := make([]byte, 64)
 	// Compute a 64-byte hash of buf and put it in h.
 	ShakeSum256(h, buf)
+	fmt.Printf("%x\n", h)
+	// Output: 0f65fe41fc353e52c55667bb9e2b27bfcc8476f2c413e9437d272ee3194a4e3146d05ec04a25d16b8f577c19b82d16b1424c3e022e783d2b4da98de3658d363d
 }
 
 func Example_mac() {
@@ -303,4 +306,6 @@ func Example_mac() {
 	d.Write(buf)
 	// Read 32 bytes of output from the hash into h.
 	d.Read(h)
+	fmt.Printf("%x\n", h)
+	// Output: 78de2974bd2711d5549ffd32b753ef0f5fa80a0db2556db60f0987eb8a9218ff
 }