encoding/base64: fix format error

Found by pending CL to make cmd/vet auto-detect printf wrappers.

Change-Id: I2ad06647b7b41cf68859820a60eeac2e689ca2e6
Reviewed-on: https://go-review.googlesource.com/109344
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/base64/base64_test.go

@@ -159,7 +159,7 @@ func TestDecode(t *testing.T) {
 
 			dbuf, err = tt.enc.DecodeString(encoded)
 			testEqual(t, "DecodeString(%q) = error %v, want %v", encoded, err, error(nil))
-			testEqual(t, "DecodeString(%q) = %q, want %q", string(dbuf), p.decoded)
+			testEqual(t, "DecodeString(%q) = %q, want %q", encoded, string(dbuf), p.decoded)
 		}
 	}
 }