Minor updates to FlushWAL blog
Minor updates to FlushWAL blog

Summary: Closes https://github.com/facebook/rocksdb/pull/2792

Differential Revision: D5715365

Pulled By: maysamyabandeh

fbshipit-source-id: 0837b93ea1d4b0a08dfb3cd0d1feb6e098ef26a4

Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb