Fix the "go get" path in the README.
Fix the "go get" path in the README.

shaunstanislaus/leveldb,mikewiacek/leveldb,shaunstanislaus/leveldb,yh453926638/leveldb,gaowenbin/leveldb,golang/leveldb,gaowenbin/leveldb,yh453926638/leveldb,mikewiacek/leveldb,golang/leveldb