Fixes sinatra example

[asmsuechan]

Hello! I fixed require path in sinatra example.

On prev codes, this error message appeared in my environment.

$ maglev-bundle exec rackup
ERROR 2702 , a RubyLoadError occurred (error 2702), no such file to load -- txn_wrapper (LoadError)
topaz 1> exit

After I fixed, sinatra app runs fine.

[jc00ke]

Thanks!