bytes: change ExampleReader_Len to use a non-ASCII string

This should help make clear that Len is not counting runes.
Also delete empty string, which doesn't add much.

Change-Id: I1602352df1897fef6e855e9db0bababb8ab788ca
Reviewed-on: https://go-review.googlesource.com/78110
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/bytes/example_test.go

@@ -430,11 +430,9 @@ func ExampleToLower() {
 }
 
 func ExampleReader_Len() {
-	fmt.Println(bytes.NewReader([]byte("")).Len())
 	fmt.Println(bytes.NewReader([]byte("Hi!")).Len())
-	fmt.Println(bytes.NewReader([]byte("Hello Gopher!")).Len())
+	fmt.Println(bytes.NewReader([]byte("こんにちは!")).Len())
 	// Output:
-	// 0
 	// 3
-	// 13
+	// 16
 }