docs: DGraph no longer use rocksdb

[thisiswhereitype]

Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev

Docs

https://github.com/facebook/rocksdb/wiki/RocksDB-Users-and-Use-Cases#dgraph

DGraph no longer use rocksdb: https://dgraph.io/blog/post/badger-over-rocksdb-in-dgraph/