Fix error in example

[andrewdbate]

Fix an example, changing single quotes to double quotes (fixes #263).

[lightbend-cla-validator]

Hi @andrewdbate,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Typesafe Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[eed3si9n]

Thanks!