bufio: fix ExampleScanner_Bytes comment, add error check

Followup to CL 51412.

Change-Id: Ic83c833e2c571cd7c8293d998ff745f181037a61
Reviewed-on: https://go-review.googlesource.com/c/go/+/183657
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/bufio/example_test.go

@@ -31,12 +31,15 @@ func ExampleScanner_lines() {
 	}
 }
 
-// Use return the most recent call to Scan as a []byte
+// Return the most recent call to Scan as a []byte.
 func ExampleScanner_Bytes() {
 	scanner := bufio.NewScanner(strings.NewReader("gopher"))
 	for scanner.Scan() {
 		fmt.Println(len(scanner.Bytes()) == 6)
 	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "shouldn't see an error scanning a string")
+	}
 	// Output:
 	// true
 }