Clean up spec tests for rspec-expectations 2.11.3 and up

Starting in Rspec Expectations 2.11.3, a deprecation warning was added
for `expect { }.should` syntax. The desired syntax is `expect { }.to`.

This commit update two instances of `expect { }.should` in the
'spec/unit/puppet/type/registry_key_spec.rb' file. As a result, there
are no more deprecation warnings from Rspec.