Fix example spec file in README
Fix example spec file in README

While following this example, I needed this change to get the tests to pass.

[The documentation for beaker](http://www.rubydoc.info/github/puppetlabs/beaker/Beaker%2FDSL%2FHelpers%2FPuppetHelpers%3Aapply_manifest_on) explains why:

> `:catch_failures` (Boolean) — default: `false` — By default `puppet -apply` will exit with 0, which does not count as a test failure, even if there were errors or changes when applying the manifest. This option enables detailed exit codes and causes a test failure if `puppet -apply` indicates there was a failure during its execution.
puppetlabs/beaker-rspec