[conformal/btcdb] c57083: Update README.md example to use leveldb.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcdb
  Commit: c570830104055febdf5c41ed7a86a7a1daaf2c76
      
https://github.com/conformal/btcdb/commit/c570830104055febdf5c41ed7a86a7a1daaf2c76
  Author: Dave Collins <da...@conformal.com>
  Date:   2014-03-17 (Mon, 17 Mar 2014)

  Changed paths:
    M README.md

  Log Message:
  -----------
  Update README.md example to use leveldb.

SQLite is now fully deprecated and has been removed.  Spotted and reported
by @jolan.

Closes #3.