Apply explict RSpec/DescribedClass rubocop rule

We settled on using this style in commit 4cbe81a1, but didn't add the
rule enforcing this style and we didn't apply it to existing code.

spec/lib/remote_census_api_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
 describe RemoteCensusApi do
-  let(:api) { described_class.new }
+  let(:api) { RemoteCensusApi.new }
 
   describe "#call" do
     let(:invalid_body) { { get_habita_datos_response: { get_habita_datos_return: { datos_habitante: {}}}} }