Fix wrong reference in README.

When talking about integration with Puppet, I suppose the link is expected to
refer/link to `rspec-system-puppet`.
Merge pull request #50 from stefanozanella/fix_readme_link

Fix wrong reference in README.
Fix typo in example test code.

Looks like `/etc/resolv.conf` has nothing to do with the test suite in the
example.
Merge pull request #51 from stefanozanella/fix_readme_example

Fix typo in example test code.