updated bug URL in readme
updated bug URL in readme

shaohua-zhang/incubator-datafu,shaohua-zhang/incubator-datafu,apache/incubator-datafu,apache/incubator-datafu,apache/incubator-datafu,apache/incubator-datafu,shaohua-zhang/incubator-datafu,shaohua-zhang/incubator-datafu,apache/incubator-datafu,shaohua-zhang/incubator-datafu