blake2b: fix typo in error message

io.EOF is expected by the test, but the error message mentions os.EOF.
Adjust it to io.EOF.

Change-Id: I1a42da8d72f96f8757319183a6bb3281a64b3a40
Reviewed-on: https://go-review.googlesource.com/65870
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

blake2b/blake2b_test.go

@@ -126,7 +126,7 @@ func testHashes2X(t *testing.T) {
 			t.Fatalf("#%d (single write): error from Read: %v", i, err)
 		}
 		if n, err := h.Read(sum); n != 0 || err != io.EOF {
-			t.Fatalf("#%d (single write): Read did not return (0, os.EOF) after exhaustion, got (%v, %v)", i, n, err)
+			t.Fatalf("#%d (single write): Read did not return (0, io.EOF) after exhaustion, got (%v, %v)", i, n, err)
 		}
 		if gotHex := fmt.Sprintf("%x", sum); gotHex != expectedHex {
 			t.Fatalf("#%d (single write): got %s, wanted %s", i, gotHex, expectedHex)