Fix README

[DenisGorbachev]

Hi Marvin, thank you for developing Fjall!

This PR contains three minor fixes:

• The example code has been wrapped in fn example (otherwise the IDEs complain that the snippet is not valid Rust because let declarations are not allowed at the top level, only in functions).
• The persist call has been fixed: keyspace.persist -> db.persist
• The type names have been fixed:
  • OptimisticTransactionDatabase -> OptimisticTxDatabase
  • SingleWriterTransactionDatabase -> SingleWriterTxDatabase