encoding/hex: fix example function name

Found by vet.

Change-Id: I556d87f853a734002f779b04ba5a3588a3117106
Reviewed-on: https://go-review.googlesource.com/28958
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/hex/example_test.go

@@ -38,7 +38,7 @@ func ExampleDecode() {
 	// Hello Gopher!
 }
 
-func ExampleDecodedString() {
+func ExampleDecodeString() {
 	const s = "48656c6c6f20476f7068657221"
 	decoded, err := hex.DecodeString(s)
 	if err != nil {