Fixed code formatting in README

Hey guys,

Just a quick patch for your README, the code examples were accidentally converted to quotes, rather than code blocks. Feel free to look at the README on [my branch](https://github.com/prezjordan/useful_duration/blob/quickfix/readme-code-blocks/README.md) to see the results of this PR.

https://github.com/prezjordan/useful_duration/blob/quickfix/readme-code-blocks/README.md

Cheers