Replaced "built on on earlier work" by "built on earlier work" in README.md

Author: yhchiang

README.md

@@ -3,7 +3,7 @@
 [![Build Status](https://travis-ci.org/facebook/rocksdb.svg?branch=master)](https://travis-ci.org/facebook/rocksdb)
 
 RocksDB is developed and maintained by Facebook Database Engineering Team.
-It is built on on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com)
+It is built on earlier work on LevelDB by Sanjay Ghemawat (sanjay@google.com)
 and Jeff Dean (jeff@google.com)
 
 This code is a library that forms the core building block for a fast